Walden Pond State Reservation is just off Route 2, 18 miles west of Boston, Walden Pond’s public beach, walking trails and reproduction of Thoreau’s cabin and cairn teem with tourists in the summer. But when the calendar — and the weather — turns in September and October, the 462-acre state park is nearly deserted. Early in the morning the half-mile expanse of Henry David Thoreau’s favorite watering hole is a great spot for open-water swimming, and open-minded thinking.

Jay Atkinson writes in the New York Times:

Thoreau was known for his thrift, and I’ve followed his example by having a raw-food energy bar for breakfast and borrowing a state parking pass from my hometown library, saving myself $5. Crossing Walden Street with my gear, I’m serenaded by twittering birds and a last, insomniac cricket. The sun is rising above the wall of trees surrounding the pond, and at the far end of the beach, beyond the shuttered pavilion and empty lifeguard stand, the first blush of autumn has appeared on the oaks and maples. A pair of flip-flops and two or three mesh bags are lined up on the low stone wall bordering the strand; I can make out the heads of several swimmers churning their way toward the far bank. There’s no one else in sight.

Stripping down to trunks, I tug on a sleeveless wetsuit, strap on goggles and zero-out the timer on my watch. Spires of mist rise from the pond, which is flat and black and still. Wading into the pebbly shallows I throw water onto my neck, make the sign of the Cross and dive in. The water is significantly colder than the air, and the gasp reflex sends a spasm down my neck and through my torso. Then I set off.

Thoreau wrote, “Time is but the stream I go a-fishing in.” Out on the pond the sharp intake of my breath echoes in my head with every pull, while each head-tilt produces a snapshot of the rising sun. Thoreau noted that his fellow Concordians once thought the pond bottomless, and staring into the milky-green depths I can understand why. A so-called kettle hole formed by a melting glacier, Walden Pond is 61 acres in total area and 102 feet deep.

In a few minutes the layer of water between the wetsuit and my skin warms up, insulating me from the cold, and I marvel at the slanted sun reflecting off the pond onto the trees. After listening to Neil Young on the drive I can hear “Hey Hey My My (Out of the Blue)” playing in my head along with this celestial light show.

Fifteen minutes into the swim I pop up to get my bearings. Coming the other way, another swimmer is thinking the same thing, and suddenly we’re facing each other, 10 yards apart and a hundred yards from shore, blinking through our goggles. It’s like running into a neighbor on another planet.

“Good morning,” he says, and I respond in kind, feeling like a character in an Antoine de Saint-Exupéry story.

Counting strokes induces a trancelike state, helping me overcome the weariness that sets in during long, cold swims. I lose track somewhere around 750 and go streaming onward, my breath coming in just the right rhythm so I don’t feel tired, only exhilarated.

At one point it seems as if I’d risen into the air above my swimming self and were looking down at a pair of churning legs and the thin white trail I’m making across the pond. It’s one of those transcendent (or perhaps Transcendentalist) moments that make all the pain worthwhile.

In 1845 the iconoclastic Thoreau built his tiny cabin beside the pond, where he grew a garden, worked sporadically as a hired man and announced his intention to “live so sturdily and Spartan-like as to put to rout all that was not life.” After some time I arrive at Thoreau’s Cove, a shallow inlet close to where he lived for two years and two months. I stand in the waist-deep water, looking up at the cairn that marks the site, and a red maple leaf drifts past, a symbol of the Canadian weather that will soon be coming our way. I snatch it up and stow it under the chest plate of my wetsuit, and resume swimming.

Nothing lasts — especially not a New England summer — and I want to wring every stroke, every breath, every passing thought and sensation from the sanctifying waters of the pond.