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.