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A careful reading of Walden; or, Life in the Woods makes it clear that Thoreau never intended his cabin to be a solitary hermitage, although fans and detractors alike often misunderstand this. It was more an author’s workshop than a fortress of isolation, and throughout his lakeside residency he often visited family and friends in Concord and entertained guests at Walden. Ice-cutters and woodcutters, anglers and boaters, and even a noisy train were as much a part of his surroundings as the lake, woods, and wildlife. He retreated to the cabin largely in order to write in a quieter setting than he could find in town and to “live deliberately, to front only the essential facts of life, and see if I could not learn what it had to teach, and not, when I came to die, discover that I had not lived.”

Thoreau’s cabin experiment was also a field test of the transcendentalist philosophy that Emerson championed. For Emerson, nature represented an embodiment of the divine, an aesthetic ideal that was best described in poetic or quasi-religious abstractions. Contemplating it was a way to transcend normal daily life and seek deeper spiritual lessons. Emerson believed that nature was “all that is separate from us, all which Philosophy distinguishes as the NOT ME,” and “essences unchanged by man; space, the air, the river, the leaf.” Such ideas still resonate with many of us today. Recognizing that satellites now cruise space and our fossil carbon emissions contaminate the air, rivers, and leaves of the entire planet, author Bill McKibben built a similar concept into the title of his groundbreaking book on global warming, The End of Nature.

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ptwo / Wikipedia

Thoreau’s equally reverent views, however, were more explicitly anchored in physical reality than Emerson’s, the product of both aesthetic and scientific sensibilities. His journals recorded minute details of the world around him, from the number of growth rings in a tree stump to the gyrations of shiny black whirligig beetles on the surface of the lake.

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I see no whirligigs here this early in the year, but they are easy to spot on a lake such as Walden when the water is still and they can gather in close, swirling clusters. They overwinter on the bottom and emerge in spring to breed, producing new generations that grow to fingernail length within a few weeks. Each beetle uses flattened legs to paddle quickly through the thin surface film, guided by compound eyes that are each divided, with one-half aimed above the water line and one-half below. Most fish leave whirligigs alone because they leak bitter chemicals when handled, and I have seen newly stocked brook trout, brazen and ignorant from life in the hatchery, snatch whirligigs from below and then spit them back out again like slippery watermelon seeds. Whirligigs often gather in groups that help to discourage predators by pooling more watchful eyes in one place, and the whirling dances within the clusters are not as random as they seem. The individuals on the perimeter are generally searching for fallen gnats, emerging midges, or anything else edible, and they emit ripples like radar to home in on struggling prey. In adult swarms, those closer to the center are more likely to be cruising for mates, using their ripples to communicate with one another and avoid collisions.

Much more has been said and written about Thoreau’s philosopher-poet side than his naturalist side, but as a scientist I am more interested in the latter. The journals that he kept from 1837 to 1861 were so full of natural history observations that they might have become a major scientific work if he had not died of a lung ailment at age 44. He probably thought so, too. Two months before his death in 1862 he wrote a letter to a friend, saying, “if I were to live, I should have much to report on Natural History generally.”

Changes come and go, whether from ice age cycles or election cycles, but life itself continues.

During the winter of 1846, Thoreau drilled more than a hundred holes through the ice of Walden Pond and lowered a weighted line to produce what may be the first map of the floor of an American lake, thereby identifying Walden’s deepest point in the western basin near his cove. In August 1860, he also sent a thermometer down in a stoppered bottle to measure the layered structure of the water column, a first formal analysis of the thermal stratification of the lake. He was amazed at the temperature difference between the upper and lower layers, and he speculated on what it might mean for the resident fish. “What various temperatures, then, the fishes of this pond can enjoy,” he wrote. “They can in a few minutes sink to winter or rise to summer. How much this varied temperature must have to do with the distribution of the fishes in it.”

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In August 1939, lake ecologist Ed Deevey made similar measurements from a rowboat and confirmed Thoreau’s reports. He also measured the stratification of the water in more detail, finding temperatures close to 79 degrees Fahrenheit (26 degrees Celsius) in the upper 15 feet (5 meters) that fell to 41 degrees Fahrenheit (5 degrees Celsisus) near the bottom. Writing in Quarterly Review of Biology, Deevey noted that Thoreau’s curiosity was “unusually fruitful when directed toward lakes,” and called him the first American limnologist, or lake scientist.

Other scientists have also used Thoreau’s observations in their own research. The Boston University ecologist Richard Primack has compared recent observations of ice-out dates, flowering times, and other signs of spring to the dates that Thoreau recorded in his journals. In Walden Warming, he used those data to show that climate change has shortened the ice-cover season by several weeks since the 19th century. And one journal entry from 1854 tripped up another friend of mine, biophysicist Charles McCutchen.

While standing beside a local stream in 1970, Charlie had noticed something resembling a fine thread on the surface that undulated crosswise to the current. After careful study, he identified it as an ephemeral wrinkle where the surface film folded inward on itself. Soon after he published his discovery in Science, however, another researcher pointed out that Thoreau had already described the same phenomenon, both accurately and more poetically. “It is interesting,” Thoreau wrote, “to distinguish the different surfaces,—here broken into waves and sparkling with light …  and there quite smooth and stagnant. I see in one place a sharp and distinct line, as if it were a cobweb on the water …  as if it were a slightly raised seam.”

As we watched the sunlight sparkle on Lake Placid, it seemed to me that Charlie relished the thought of being scooped by Henry David Thoreau.

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When I return to Walden in August, my students Rory and Elliott carry our two canoes to the boat launch and lash them into a makeshift catamaran. It is easy to see that the lake has changed since Thoreau’s time in ways that he and Emerson would probably dislike. The adjacent beach is packed with bathers, and although the water is still clear, a faint greenish tinge warns of potential trouble.

Analyses published in 2001 by the United States Geological Survey showed that surreptitious urine releases by swimmers had approximately doubled the summer phosphorus budget of the lake. Phosphorus, whose elemental symbol is the letter “P,” is a key structural atom in cell membranes, energy-storing molecules, and genes, and it is therefore a common currency in the world’s food webs. All living things, ourselves included, consume it in food and release it in waste molecules that other organisms may later use. Humanity’s new role in the Walden Pond ecosystem as a key source of pee-P for algae is also reflected in the results of sediment core studies that were conducted in 1979 by University of Wisconsin researcher Marjorie Winkler and in 2000 by Canadian ecologist Dörte Köster and colleagues. They found that distinctive phosphorus-loving species have dominated the planktonic algal community since the early 20th century. My students and I have come here now to consult the sediments for an update on the status of the lake and to more closely examine its climatic history with an eye toward the future.

A middle-aged man with a fishing rod in his hand pauses to ask if I know where the lures-nagging weed beds are. A fibrous alga, Nitella, forms a ring of matted meadows on the lake bed at depths between about 20 and 40 feet (6 to 13 meters), but darkness prevents it from colonizing the deeper places farther offshore. Thoreau mentioned it in Walden, calling it “a bright green weed (that) is brought up on anchors even in winter.” It is common in clear-water lakes and resembles a plant, but it lacks flowers, seeds, or vein-bearing stems and roots. Unlike true aquatic plants, its ancestors remained within an entirely different kingdom of life, Protista, that is dominated by single-celled species. The Nitella meadows in Walden Pond divert dissolved phosphorus away from the microscopic algae of the plankton and trap it on the bottom. Like the continuous flush of groundwater, they help to keep the lake clear, but ecologists worry that any further clouding of the water by overfed plankton might shade them out and tip the scales in favor of pond scum.

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To this fisherman, however, the Nitella seems to be more of a nuisance than a blessing. I ask what he hopes to catch. “Rainbows and browns,” he says. Neither species lived here in Thoreau’s day. Rainbow trout are native to the western United States, and brown trout were brought to North America from Germany during the 19th century. Despite the lake’s status as a revered symbol of wilderness, county officials had it poisoned with a pesticide, rotenone, in 1968 in order to remove “trash fish” such as the pout and pickerel that Thoreau once knew and to make way for nonnative game species.

If he were to plunge his hands to the wrists in it, he could touch plankton that also brushed Thoreau’s hands during swims a century and a half ago.

A young free-diver approaches and shows me a GoPro video on his cell phone. In the video, his hands follow a guiderope into murky darkness in the 100-foot basin. According to him, the deepest part of the lake is surrounded by steep ledges, and huge snapping turtles lumber around the margins of the pit like dinosaurs. “There’s no light down there,” he explains, “so you can’t tell where the bottom is. Sometimes I do a headplant into the mud because I can’t see where I’m going.” Fortunately, our coring site will be in an adjacent basin that is—hopefully—less pitted with headplants.

As we paddle out to the center of the lake a bald eagle swoops low overhead, perhaps scanning for trout. After the huge bird flaps back up and over the tree line, my attention aims downward, too. Beneath us lies an extension of the landscape that mirrors the underbelly of the iceberg that formed it. Thoreau identified the 100-foot hole at the west end of the lake and a 55-foot (16 meter) basin at the east end near the swimming beach, but he missed a third one midway between them. The USGS scientists found it only a decade ago, measuring depths close to 65 feet (20 meters) near the center of it. That is where we are headed now.

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Rory and Elliott toss two anchors and draw the lines tight while I retrieve a conical net that I have been towing behind us. The mesh is finer than that of a nylon stocking, and it sieves the dilute broth of plankton beneath us. When I hold a glass vial of the catch up to the sky, I see creamy flecks dancing like dust motes in the sunlight. These shrimplike copepods and cladocerans are the main prey of Walden’s fingerling fish and minnows. They use their tapered abdomens as rudders and swim by paddling with multiple pairs of jointed limbs while additional limbs also strain the water for microscopic algae. A healthy population of zooplankton (animal plankton) can filter the entire volume of a lake within days, a testament to the rapid growth of the phytoplankton (plantlike plankton) they graze on. I tip most of these animals back into the lake and feel sorry for the few I must keep as specimens even though I have already killed many of their kind every time I gulped a mouthful of lake water or toweled off after a swim.

Miguel Vieira / Flickr

All around me, trillions of living specks such as these are feeding, breeding, dying, and ultimately sinking to the bottom. Joining them in the gentle flurry of debris are leaves, twigs, and puffs of pollen from the forest. Mushroom spores, insect wings, and translucent grains of beach sand from the shore. Genes and bones from fish and turtles, and the gleaming glassy shells of microscopic diatom algae. I lean over the gunwale to look through the wavering halo of sunbeams that surround my silhouette, imagining the detritus of life settling like snow beneath me. Each successive layer represents a page in the history of the lake and its surroundings. When our free-diving friend next plants his head in the soft brown ooze of the main basin, his scalp will push through decades of accumulated crud. If he were to plunge his hands to the wrists in it, he could touch plankton that also brushed Thoreau’s hands during swims a century and a half ago.

We deploy the smaller of two core samplers first, in order to confirm that we are positioned far enough from the Nitella meadows to avoid clogging the core barrel with fibers. Rory and Elliott lower the sampler hand over hand until they feel the line go limp. When it splashes aboard moments later, Rory holds it upright to avoid disturbing the loose, flocculent surface layers. The core is as long as her forearm and resembles a tube of chocolate pudding. No sign of Nitella here, so we stow the first sample and lower a longer, heavier, homebuilt device overboard. This one is equipped with a counterweight that feels the bottom and triggers a release mechanism just before the base of the core barrel meets the mud. Moments later, 33 inches (84 centimeters) of lake history break the surface.

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The results of previous coring studies suggest that this sample represents about 1,500 years, which carbon-14 dating of the mud will later confirm. A band of sediment that was deposited during Thoreau’s lifetime lies 8 to 9 inches (20 to 24 centimeters) below the surface layer of the core. I can link the two strata with the span of two hands, a distance that will eventually shorten down there on the lake bed as new, watery mud gradually compresses under the weight of future layers. Thoreau’s writings easily draw your imagination back to the 19th century, but a sediment core such as this pulls you even deeper into the past by encouraging you to ask “what happened before that?” Seeing so many relics of Walden’s yesteryears stacked one atop the other in this manner exposes self-centered views of history for what they are, reflections of our own minds that obscure our momentary positions in an open-ended river of time.

I return to Walden again in December 2016. The day is unseasonably warm and windless, and the reflected images of clouds are sharp and clear as they glide slowly over the smooth surface. During the past year my students and I have been busy analyzing samples from our cores, but rather than sample the lake today I simply want to sit beside it.

In Walden, Thoreau wrote of his desire for a fanciful “realometer” to cut through the “mud and slush of opinion, and prejudice, and tradition, and delusion …  to a hard bottom …  which we can call reality.” I am here to consult my own version of a realometer—this beautiful lake with its ancient sediment archives and the larger perspectives on life that they inspire.

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The low water level has exposed a sandbar at the mouth of the cove, inviting me to walk on it. I hunch down, lean over the water’s edge, and let my eyes explore it layer by layer. The surface lies as still as the air, and bright sunlight flickers on and off through gaps in the mirrored clouds. Refocusing on the bottom, I scan the smooth pebbles of gneiss and quartzite amid the sand and imagine them tumbling in glacial rivers. Leaning closer, I wait until the water itself comes into focus and just barely make out a tiny speck of a copepod motoring about in search of a meal or a mate.

For the first time in the history of the planet a species has produced an entirely new kind of waste.

I wonder how it would feel to sit here with Thoreau, staring together into Walden Pond. Would we see the same things in it? Probably not, but I suspect that we would enjoy sharing our impressions nonetheless. “Time is but the stream I go afishing in,” he wrote in Walden. “I drink at it; but while I drink I see the sandy bottom and detect how shallow it is. Its thin current slides away, but eternity remains.” I feel much the same today. I let my imagination sink down to the submerged sediment layer of this present moment, then deeper still.

There are layers upon layers of stories stacked under this lake, and any individual increment of mud is just one of many pages in the epic of human existence. It reminds me that all lives are finite and makes me feel less alone in my own encounters with mortality. The long geological history preserved here reveals a deep human connection to the natural world that also comforts me, one that philosophers such as Emerson who considered people to be separate from nature might not have fully appreciated. Untouched wilderness never really existed in North America, at least not since the large mammals vanished, and a Walden without Homo sapiens somewhere in the picture might be cleaner but also as artificial as a swimming pool. Envisioning the sediment records beneath the reflections helps me to clarify this truth and my own connection to the world in ways that words alone cannot.

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Echo soundings recently obtained by the Salem State University geologist Brad Hubeny suggest that the deposits beneath the eastern basin of Walden Pond are about 20 feet (6 meters) thick. Imagine driving a core barrel all the way through those sediments and then leaning the core upright against the side of a two-story house so the top stands level with the eaves. Now imagine climbing a ladder to measure the entire length of that column, not in units of feet and inches but of lifetimes, each one lasting, say, a conservative 60 years.

To get used to those unusual temporal units, consider some familiar time periods in these terms. Two and one-half such life spans separate us from Thoreau, for example, and only four separate us from the American Revolution. Six or seven life spans take us to the arrival of the Pilgrims in Plymouth and eight or nine take us to the first landing of Columbus in Hispaniola. For many people, those few life spans represent the history of America, but the imaginary sediment column puts that misconception into clearer perspective. It represents more than 200 successive human lives.

Alex / Wikipedia

Starting near ground level, a thumb when pressed against the first increment of core history might span the lives of the earliest visitors to Walden Pond who arrived after the kettle lake formed, perhaps 13,000 years ago. Stone spear points and other artifacts unearthed in the Concord area and pollen records from other New England lakes suggest that they hunted caribou on what was then a mosaic of tundra and spruce thickets much like those in Arctic Canada today. Two more thumb-widths encompass the lifetimes of the children and grandchildren of those early hunters who now join us in spirit on this sandbar.

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Continuing in like manner layer by layer, life by life, we approach chest height in a 3,000-year period of hunting and gathering half again as long as the stretch of time between the present day and the birth of Christ. This represents the long “PaleoIndian” period of New England’s past that began with the retreat of the last ice sheet.

Slightly more than 2 feet (70 centimeters) higher above the ground we are roughly 75 life spans into the story, when warm, dry climates supported a fire-prone mix of savanna grasses, pitch pine, and oak. Some of the oak pollen in this mud may have made a local deer hunter sneeze, spooking a buck he had hoped to take with a stone-tipped dart when the animal came to the lake to drink. As a member of the local “Archaic” culture, he would have dined often on deer, wild turkeys, and acorns in the surrounding forests without ever having heard of the maize, beans, and squash that would not arrive for another seven millennia, longer than the history of the Egyptian pyramids.

Three-quarters of the way up the core, we reach sediments that were deposited when people of the late Archaic to early “Woodland” cultures used some of the earliest clay pots while they camped and cooked beside the lake 3,000 years ago. About 2 feet (70 centimeters) from the top we are 16 lifetimes away from the present. The 1,000-year-old sediments there contain charcoal from the seasonal burning of forest underbrush and maize fields in a place that only became known as “Concord” a few short inches ago on the mud timeline.

Now, while the imagery is still fresh, ask yourself if people are part of the natural order of things at Walden Pond.

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What names did the first maize farmers give to this lake, or the Woodland potters before them, or the Archaic deerhunters before them, or the PaleoIndian caribou hunters before them? What did they discuss over breakfast on the shore—if they had it—or daydream about while watching reflections from the sandbar at the mouth of “Thoreau’s” cove?

We will never know for sure, but other insights from the Walden realometer help to steady me in this particular moment. Changes come and go, whether from ice age cycles or election cycles, but life itself continues. I am a spark among many in that ancient, evolving flame. I glimpse a reassuring measure of eternity in it, too.

Today’s widespread nutrient pollution, species invasions, extinctions, and soil erosion rival some of the most dramatic environmental disruptions of the geologic past. A growing number of scientists agree that unique signs of our modern connections to nature in the aquatic sediments of the Earth are extensive enough to merit a new “Anthropocene” name for our present epoch. However, they disagree over the date that best represents the transition.

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For some researchers, the postglacial extinction of the large mammals makes a reasonable signpost. However, the cause of the die-off is still debated. In addition, it was not truly global in extent because many large mammals persist in Africa, and it occurred over many centuries, so it doesn’t make a uniformly clean break in the geologic record.

Others prefer to put the onset at the dawn of agriculture in the Middle East about 10,000 years ago. As farms spread around the world, so too did deforestation, soil erosion, and the environmental effects of urbanism, as Mayan clays and Crawford Lake sediments attest. However, those changes, like the mammal extinction, were not globally synchronous and omnipresent.

Any individual increment of mud is just one of many pages in the epic of human existence.

In 2015, British geoscientists Simon Lewis and Mark Maslin proposed two candidates for a “golden spike” event to define the start of the Anthropocene. One was a dip in CO2 concentrations in polar ice cores that dates to the early 1600s when imported diseases killed millions of Native Americans, including many of the Massachusett peoples. So much former cropland returned to forest as a result of the plagues that the sequestration of carbon in new wood and foliage apparently reduced the CO2 content of the atmosphere.

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Their other candidate was the early 1960s, when the world was most heavily contaminated with fallout from the atmospheric testing of thermonuclear weapons during the Cold War. The cesium-137 peak in lake deposits is so widespread that scientists already use it as a timemarker in sediment cores, as we did at Walden. The mud 4 inches (10 centimeters) below the top of our long core is unusually radioactive, according to a technician at the Museum of Minnesota who analyzed the isotopes for us, a unique signature of the dawn of the atomic age if not the Anthropocene. For the first time in the history of the planet a species has produced an entirely new kind of waste, one that is not simply a rearrangement of preexisting elements but new atoms forged in the violent hearts of artificial stars.

Canadian ecologist Alex Wolfe and colleagues recently summarized the diversity of changes that are revealed in cores from remote backcountry lakes worldwide. Artificially generated nitrogen compounds suffuse the recent layers because fossil fuel combustion and the industrial production of fertilizer now dominate the global nitrogen cycle. Arctic lakes that have recently lost their summer ice lids now support planktonic diatoms who are leaving their remains in the most recent sediments for the first time in thousands of years, strong evidence that the recent warming is unusual and not due to natural climate cycles. And a combination of nitrogen pollution and warming seems to be causing a rise of chrysophyte algae in high altitude lakes from Alberta to the Andes.

Our profound impacts on the natural world occur because we are profoundly connected to it. Unfortunately, those connections are often hidden by the limitations of our senses, as a lake’s reflective surface obscures its depths. We can’t easily see the teeming atoms of air, water, soil, and organisms that make up our bodies, nor notice what happens to them when they leave us as waste. Nonetheless, these elemental connections hitch us to all life and the Earth itself whether we recognize them or not, just as they did for our predecessors and will continue to do for our descendants. To more fully recognize and respect such connections is one of many challenges we face as the Anthropocene epoch unfolds.

Thin clouds slip quietly over the face of Walden Pond while I prepare to leave the sandbar at the mouth of Thoreau’s cove on this December afternoon. I hunch down and brush my fingers through the mirror one more time in a ritual farewell.

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We are not separate from nature. We are nature, an ancient truth that can perhaps most clearly be seen through the eyes of lakes.

Curt Stager is a professor of natural sciences at Paul Smith’s College in New York. He has published in periodicals such as Science, National Geographic, and The New York Times.

Adapted from Still Waters: The Secret World of Lakes by Curt Stager. © 2018 by Curt Stager. Used with permission of the publisher, W.W. Norton & Company, Inc. All rights reserved.

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Lead image: Stefan Estassy / Folio / Getty Images

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