The Deep Sea

“Yet they’re quite evolved”—they’re

“Yet they’re quite evolved”—they’re invertebrates, but the larvae have spinal cords. Synoicum Adareanum

At depths of 30 to 50 feet, forests of kelp, with blades more than 10 feet long, create a sober, imposing scene. Farther down, we meet giant sea stars: At 15 inches in diameter, they’re much bigger than those in warmer seas. Then come the giant sea spiders. They’re arthropods, like insects and spiders on land, and they’re found in all the world’s oceans, but in warmer waters they’re rare and tiny, nearly invisible to the naked eye. Here, as in the Arctic, the sea spiders can span a foot or more. Yet their bodies are so small that their internal organs extend into their legs.

Below 165 feet, the light dims and we see no kelp or other plants. Instead, the seafloor is covered with thick carpets of feather hydroids (colonial animals related to corals) and with thousands of scallops. The scallops are four inches across but may be 40 years old or more—things grow slowly in the Antarctic. At these depths we also notice feather star crinoids, close relatives of sea stars, which snag particles of drifting food with up to 20 undulating arms. Crawling and swimming among them are giant isopods that resemble beetles. a brown fuzzy clam-like thing in a bright blue ocean a rigid crustacean walking on rock on the ocean floor an orange sea star on a bed of green sea plants on the ocean floor a yellow long-legged spider under the ocean in bright blue water Extreme conditions beget extreme animals. Clockwise from upper left: Three inches long, this icebound Antarctic scallop is probably decades old—growth is slow in the extreme cold. An isopod looks like a pill bug—and rolls up when threatened— but is nearly fiv...

At 230 feet, the limit of our dives, the diversity is greatest. We see gorgonian sea fans, shellfish, soft corals, sponges, small fishes—the colors and exuberance are reminiscent of tropical coral reefs. The fixed invertebrates in particular are enormous. Well adapted to a stable environment, these plantlike animals grow slowly but, it appears, without limit—unless something disturbs them. How, we can’t help wondering, will they respond as climate change warms their world?

(Read more: Fast-Growing Moss Is Turning Antarctica Green)

As we ascend to the surface, the biodiversity diminishes. The shallower waters are a less stable environment: Drifting icebergs and sea ice scour the seafloor, and the seasonal freezing and melting of the sea surface, which removes freshwater from the ocean and then returns it, causes dramatic swings in salinity. But there is still plenty to occupy the eye. Microalgae cling to the ceiling of ice, turning it into a flamboyant rainbow of orange, yellow, and green. The ceiling is really more like a chaotic labyrinth, with layers of ice at different levels, and we pass through them slowly, cautiously. One day as I’m nearing the hole, I see a mother and baby seal plunge through it. I watch them for a long, envious moment as they move effortlessly through this fairy landscape.

On another day, while I’m desperate for distraction from the cold, Gentil calls my attention to a field of tiny, translucent anemones hanging from the floe. They’re rooted a few inches deep in the stonelike ice, and their tentacles, pierced by the sun and waving in the current, are sharp and shiny. In all my research I’d never heard or read of such animals. They’re mesmerizing.

The scientists back at the French base, looking at our pictures, say they’d never seen our ice anemones either. At first we’re very excited; we think we’ve discovered a new species. Later we learn that scientists working in the American sector had described the animals two years earlier, based on photographs and samples taken with a remotely operated vehicle. We’re disappointed but still proud, because we’ve seen these amazingly delicate creatures live, with our own eyes. a seal mom and her pup swimming together under the surface ice chunks

A Weddell seal accompanies her pup on a swim beneath the ice. When the juvenile is full-grown, it will be its mom’s size: about 10 feet long and weighing half a ton. These placid seals stay close to the coast, breathing air through holes in the ice. Leptonychotes Weddellii a gray rotund fish hiding behind large kelp plants a translucent sea plant with tentacles the color of ice against a dark blue background Left:

A wary icefish takes cover in a kelp grove. These bottom dwellers have antifreeze proteins in their blood that help them withstand temperatures below 29°F. There are at least 50 species of icefish in the frigid waters of Antarctica. Family Nototheniida... Right:

Body stowed inside the ice floe, an anemone lets its tentacles dangle in the dark water. Marine biologist Marymegan Daly says it’s the only anemone species known to live in ice. Scientists can’t say how it penetrates the ice—or survives there. Edwardsiella Andrillae

The waters under Antarctic ice are like Mount Everest: magical, but so hostile that you have to be sure of your desire before you go. You cannot go half-heartedly; you cannot feign your passion. The demands are too great. But that’s what makes the images you see here unprecedented, and the experience of having taken them and of having seen this place so unforgettable.

An octopus jets above a seabed packed with life. Antarctica has at least 16 species of octopuses. All have a specialized pigment in their blood called hemocyanin, which turns the blood blue and helps them survive subfreezing temperatures. Pareledone sp.

After 36 days we felt we’d only begun to plumb it. The trip was so intense—the work so hard and exhausting, the sleep each night so deep—that in memory it seems to fuse into a single, 36-day-long dive. Our feet and hands froze, but our emotions were on a perpetual boil.

One dive toward the end stands out in my heart, not for the animals we saw but for the location. At home in France, looking at the Dumont d’Urville map, I had dreamed about it. Where, in this century on this Earth, can you be truly alone? Where can you see something no one has seen before? On the map I marked the Norsel reef, a tiny island more than seven miles offshore from Dumont d’Urville. In winter it’s icebound.

By the time our helicopter flew over it, Norsel was in the open sea, a spire of rock just piercing the surface of water more than 600 feet deep. It was topped with a little ice cap. When the chopper dropped us, we were surrounded by ocean and giant icebergs—and well aware of the privilege of being where nobody had ever dived.

Summer was coming, and it was a mild, almost balmy day, around freezing. But the water was still below 29°F. Blanche, the doctor, activated the chronometer: He gave us three hours and 40 minutes. Then we were off, for another soak in another world. French biologist and photographer Laurent Ballesta covered coelacanths for the March 2011 issue. His next expedition will take him back to French Polynesia to swim at night with gray reef sharks on the hunt.