Belize's Great Blue Hole was popularized by underwater explorer Jacques Cousteau in a 1971 episode of his series, The Undersea World of Jacques Cousteau. But what was at the bottom of the 410-foot-deep ocean sinkhole, which formed as a limestone cave thousands of years ago before rising sea levels submerged it, remained unknown. Nearly half a century later, Cousteau's grandson, Fabien Cousteau, answered that question with the help of billionaire Richard Branson and National Geographic Explorer. They launched an expedition to the Great Blue Hole—which, at nearly 1,000 feet across, is the largest ocean sinkhole in the world (though not the deepest)—in December, and now they're out with a preview of what they found, Newsweek reports. Among those findings? Hundreds of conchs, old scientific equipment, a GoPro with footage still intact—and the bodies of two divers.
"We ... encountered the resting place for two of the divers who’ve been lost in the hole," submersible pilot Erika Bergman says. "We notified the local authorities, and everyone agreed to leave them undisturbed. They are at peace." All of those things were found beneath the hydrogen sulfide, or H2S, layer, which sits at a depth of 290 feet. H2S, a toxic gas, dissolves in water; sea life cannot survive beneath the H2S layer and won't pass through it. "Below the H2S, there’s no oxygen and anything that falls down there now is preserved," Bergman explains. Also at the bottom of the hole? Unidentifiable tracks that, Bergman told CNN last month, remain "open to interpretation." The researchers have released a minidocumentary about the expedition. (The Great Blue Hole may hold the secret to the Mayan collapse.)