They spent 28 days under the sea—and found another Earth (2024)

I grew up along the Mediterranean coast, in the south of France. My first swimming, snorkeling, and scuba diving took place on the Riviera. As time passed, and the Mediterranean became my professional home base, I watched this torrentially visited coast be ravaged by unbridled development—and I also saw, at depths of more than 150 feet or so, worlds that still seemed intact. Until recently, however, I’d gotten only brief glimpses of them. When you’re scuba diving, it takes four to six hours just to ascend from such depths; you have to decompress slowly to avoid dying from the bends. So time on the bottom is frustratingly short, usually only five or 10 minutes.

In July 2019 we changed that. For 28 straight days my four-man team lived in a cramped, pressurized habitat on a barge in the Mediterranean, breathing a high-pressure mix of helium and oxygen, descending to the seafloor each day in a diving bell. We worked like saturation divers in the offshore oil industry—but unlike such divers, who typically are tethered to a bell by an umbilical, we put on scuba gear, with rebreathers that filtered carbon dioxide from our exhalations. That meant we could explore the bottom freely—for hours, not minutes.

Because both the bell and the habitat (and the bathroom in between) were kept at the same high pressure as the seafloor—as much as 13 times surface pressure—we didn’t have to decompress each time we ascended. Instead we decompressed just once at the end of the mission, for nearly five days, before opening the heavy metal hatch of our capsule and breathing the open air once again.

On July 1, 2019, off Marseille, that hatch clanged shut behind us after we entered the diving bell, all geared up in our red suits for our first elevator ride down. It felt as if we were in a vessel taking us to the moon. At the seafloor, as we exited through a bottom air lock and swam away, the feeling was incredible: We were deepwater aquanauts leaving our link to home. I took a look back at the bell, fading in the blue. On that first dive, around 226 feet down, we kept it in sight.

Humans have crisscrossed the Mediterranean for millennia, but the bottom is a world even less known than our well-mapped moon—and unlike the moon, it’s full of life. We wandered slowly, without hurry, in Calanques National Park, where Jacques Cousteau and Louis Malle came in the 1950s to make The Silent World, a film that introduced a generation to undersea life. On that first dive we saw an animal I’d seen only once before, a decade earlier, for an instant: the veined squid. A pair of them were mating right in front of us. The male passed under the female, and their tentacles intertwined; the male slipped his lower arm, which carries the sperm, under the female’s mantle. Seconds later, the female swam into a small cave and hung long clusters of fertilized eggs from the ceiling.

These squid live only three years or so and get just one chance to reproduce. Once in a lifetime—and we were there. As far as I know, the behavior had never been documented before. For our first day, it seemed a good omen.

In the 1960s Cousteau pioneered the use of seafloor habitats along this same coast, living on the bottom for weeks at a time. We had a great advantage: We weren’t tied to one spot. In 28 days our barge, towed slowly by a tugboat, traveled more than 350 miles, from Marseille to Monaco and back. We dived in 21 places.

In our 55-square-foot habitat, the four of us—Yanick Gentil, Thibault Rauby, Antonin Guilbert, and I—were voluntary prisoners. There we rested, ate meals passed through a small air lock by the crew on deck, and waited for the next dive. The dives were our escape. Every day we endured violent contrasts: From the stifling heat in the cramped steel container to the bone-chilling immensity of the depths, from mind-numbing inaction to vital vigilance, from desperation and depression to ecstasy and euphoria. At the end of each day, we were exhausted. And we couldn’t wait to do it all over again.

The gas mix we breathed, 97 percent helium and 3 percent oxygen, prevented nitrogen narcosis and epileptic convulsions at depth. But it turned our voices into high-pitched, almost incomprehensible quacking, forcing us to communicate through microphones and software that adjusted the sound back to (almost) normal. The helium had another curious side effect: It’s such a good heat conductor that it chilled us from the inside, draining our body heat with each breath. I have dived deep under the ice in Antarctica, in water that was below freezing, but I felt much colder here in my home sea, where even at depth the temperature was a steady 57 degrees Fahrenheit.

We picked places to explore that we knew to be beautiful and flourishing. Coral reefs are rare in the Mediterranean; what it has instead, at depths between 230 and 400 feet, are “coralligenous” reefs, built by red algae. They secrete hard calcium carbonate foundations that are reinforced by some animals—worms, mollusks, corals—and gnawed at by others, such as sponges.

The constant struggle creates a textured world of nooks and crannies in which more than 1,650 species can find their niche. Sparkling pink swallowtail perch are abundant, but I’d waited years to see their far more elusive cousin: the parrot sea perch, which is slenderer, with a larger eye and a distinctive tail. Off Le Lavandou, I made what might be the first photograph of this little fish alive. Things like that convinced me there was a point to all this crazy effort.

The photographs here show just a few of the creatures we met in four weeks, at depths as great as 466 feet. We saw strange shapes, bizarre attitudes, deceitful intentions. We saw a gorgonocephalid, for example, named after the Gorgons of Greek myth, who had hair of coiled serpents and the power to petrify those who looked at them. This basket star is harmless, barely four inches in diameter when coiled—but as I watched, it slowly unfurled its endlessly branching arms to 10 times that width. When these bewitching creatures meet, they often intertwine those arms in delicate caresses. Why? That remains a mystery, because like sea stars, they reproduce at a distance, without touching, discharging their gametes into the current.

At 466 feet, only one percent of the sun’s light penetrates the gloom. But there are no longer any plankton either, so the water is clear, and even in the gloom it’s possible to see and photograph vast spaces. Just off Villefranche-sur-Mer, where the Alps extend under the Mediterranean and the seafloor drops off steeply, I was suddenly afforded views like a mountaineer, onto another Earth, right next to our own.

Those two worlds are connected. The seafloor mud we sampled contained pesticides, hydrocarbons, carcinogenic PCBs. Surface waters were alive with noise and human activity. Fleeing such pressure, the large animals we met—monstrous monkfish, dragonlike congers, tanklike lobsters—all seemed to have retreated to greater depths. There, the Mediterranean is still alive. Its heart is still beating. But what sort of future will we offer it?

Photographer and biologistLaurent Ballestahas dived among frenzied sharks in Polynesia, under ice in Antarctica, and with coelacanths off South Africa—and told the tales forNational Geographic.

This story appears in the May 2021 issue ofNational Geographic magazine.