We motor through the clammy mists veiling the coastline. Visibility comes and goes but mostly goes, forcing us to home by sound—the dull thud and whoosh of waves, the piping calls of black guillemots, or sea pigeons, as the local Newfoundlanders call them. We bump in an inflatable Zodiac through shallows and around rocky shoals, taking slaps of water aboard. Now and again an explosion of whale breath detonates nearby, startling us. We can’t see the whales, though we can smell their breath and hear the bass whistles played by the sea air across the flutes of their blowholes.

We are laboring under conditions composed of part sea, part air, part land, a confluence as tricky as a busy urban intersection at night without the benefit of headlights or traffic lights. We are struggling with the wind, the fog, the slipperiness of the boat, and the presence of foundering objects in all directions. This is the season of the annual migration of icebergs, a spectacle as grand as the exodus of wildebeest through the Serengeti, with the added potential of collisions between us and the migrators.

Newfoundland’s drifting nomads calved from their mother glaciers in Greenland two or three years ago and from that icy world they have meandered south at an average pace of less than half a mile an hour, captained by the wind, steered by the currents, following the path of least resistance along a route known as Iceberg Alley. The ones we’re seeking to avoid now have survived the ice jams to the north in Baffin Bay, the bottleneck of the Davis Strait, the dead-end fjords of Labrador.

Many lost the race just upcoast of here inside the labyrinthine claw of Notre Dame Bay. Yet five hundred to a thousand are still on the move, destined to cross below forty-eight degrees north latitude, to dip into the warm waters of the Gulf Stream. A rare few, perhaps two a century, will endure all the way to Bermuda, seventeen hundred miles south. We are hoping to meet and film a few of these icy nomads, but only from a respectable distance and in clearer air. So we creep, idle the engine, listen, and creep some more.

It’s summer, dark, gloomy, cold—lourd, as the locals say. The damp and the wind disarm our parkas so that we shiver even in pockets of sunshine. Yet our discomforts are largely forgotten in this numinous theater whenever white eyelids of fog open to reveal an Irish-green fjord, a hank of blond kelp bound to the seashore and tongued by waves, the clattered bones of a ghost village sagging to driftwood. In one blink of mist a muscular cliff materializes, tagged with the orange tattoos of crustose lichens hundreds of years old.

And then the curtain opens for good and the sun burns a sideways lantern on the island of Newfoundland and its melancholy set of blue rocks, wind-shorn mosses, black pools, growling waves. Arctic terns, Sterna paradisaea, wheel on the air, their sharp kip-kip-kip calls ricocheting from the coves. We take it in, swiveling on the pontoons of the Zodiac—when another eye opens, not in the fog but in the cliff, a sea cave arched like Neptune’s nose. Although there’s much else of interest to investigate, we find our attention and therefore the Zodiac reeled into the cavern, into the dark coldness scaled with barnacles and the moist breath of the sea.

We cut the engine and our wake flicks away, climbs the walls, returns, stirring the black water, raising its depths. At first we see nothing, and then we do, just below the surface, milky bells pulsing upside down, as soft as cellophane bags frayed at the tentacles. It’s a swarm, or smack, of moon jellyfish, Aurelia aurita, hundreds, maybe thousands of individuals carried into the cave by the tide. They are translucent, and then pearlescent, depending on the angle of light coming through the nostril of the cave. Nestled inside the clear tissues of their bells are violet-colored gonads in the shape of shamrocks, some ripe with eggs. Without hurry or direction, these four-leaf clovers, encased in their gelatin bells, tumble in slow motion, jetting as the domes contract, pulsing as they relax, collapsing when two jellies collide, then blossoming open to jet and pulse again, the tireless, mobile heartbeat of the deep blue home.

We can’t see it, but the moon jellyfish, for whom travel and feeding are synonymous, are hunting the minute prey of the underwater cosmos—the microscopic planktonic larvae of fish, crustaceans, and mollusks. The pulsing motion of the jellies not only drives them forward—or backward or upward or down—but also drives a current that sucks their prey toward them. This deceptively gentle technique bypasses the sensory alarms of the floating plankters.

It’s a graceful scene of death and destruction, this unhurried martial art in a black cave inside Neptune’s nose. So we drift, hypnotized by invisible violence. We might flow this way forever, growing stiller and colder—except our dream is interrupted by a splash as quiet as a sniff, followed by a head so ancient, so enormous, its black skin mottled with white, its outlines smudged, like a charcoal smear, across the imperceptible canvas of black water and pearly medusae. There’s no fog in the cave, yet a vapor has lifted, a screen between worlds, and in the blink of a jellyfish’s pulse we have seen backward through epochs.

It’s a sea turtle, and unmistakably a leatherback sea turtle, with ridges the length of the back, flippers the size of oars, and a head like a draft horse’s, wearing a jellyfish mane. We can hear the whistle of the inhale through its tiny nostrils.

It’s not surprising to see a leatherback up here on the edge of the ice, since this is among the most traveled of all vertebrate species: perpetually on the move along jellyfish highways between the tropics and the high latitudes. Yet it is a wonder to see a reptile at home in this cold realm. And not just any reptile but one bigger than we can imagine, heavier perhaps than the colossal saltwater crocodiles of tropical Australasia. Because of the size, it’s likely a male, perhaps nearing the known size limit for his species of eight-and-a-half feet and two thousand pounds. He is big enough to overcome the defining characteristic of the reptiles, big enough to become effectively warm-blooded and survive the realm of the icebergs.

Here in water registering between forty and forty-six degrees Fahrenheit, he maintains an internal temperature of as much as seventy-eight degrees, a feat managed with the aid of an insulating fat layer and a countercurrent heat-exchange system built between the closely aligned arteries and veins in the flippers. The warm arterial blood from his core warms the cool venous blood at his extremities before returning back to his heart, keeping the turtle as a whole warm. This warming system keeps his muscles limber and, in combination with the hydrodynamic ridges along his back, enables him to swim at speeds greater than other reptiles, in excess of twenty miles an hour, with enough maneuverability to outflank even the swiftest shark predators.

Placid black eyes half open in their nearly vertical lids observe us. Then he sinks, the black water closing over his oars, the jellyfish moons orbiting the drain of his exit.