Tonight, filmmaker James Cameron set a record for the deepest solo dive ever. We’re not kidding.

He reached a depth of 35,756 feet (10,898 meters) Sunday afternoon aboard a submersible called the Deepsea Challenger. Cameron and his crew built the craft nearly from scratch to achieve a feat accomplished by only two others — to reach the bottom of the Challenger Deep in the Marianas Trench, an area 120 times the size of the Grand Canyon and a mile deeper than Mount Everest is tall. Right now, we only have very educated guesses about what Cameron may find there, but he built the craft to get there quickly and as safely as possible and to collect a good amount of data when it reached the destination, so there’s a very good chance that he’s advanced our knowledge about what lies at the bottom of the ocean greatly.!/DeepChallenge/status/184003672857001984!/DeepChallenge/status/183999546009993217!/DeepChallenge/status/184007939516940290!/suzymusing/status/184007207375998976!/PaulGAllen/status/184010148476502016!/PaulGAllen/status/184016623898857473!/PaulGAllen/status/184018502556979202

And finally, the historic tweet from Cameron himself.!/JimCameron/status/184036733959143425

You’ll notice the presence of SpaceX co-founder Paul Allen in those tweets. Allen, one of the founders of Microsoft and the investor behind Burt Rutan’s SpaceShipOne that won the Ansari X Prize, was following the dive with special underwater radio equipment from onboard his yacht and providing “play by play” to the world through Twitter. After a short time, during which Cameron planned to survey the area and locate any undersea life that might have been drawn to an unmanned “lander” dropped at the location earlier, Cameron headed back to the surface.!/PaulGAllen/status/184080394105270272!/PaulGAllen/status/184095201709662208

And, a bit over 5 hours after he gave the command to begin, Cameron reached the surface.!/DeepChallenge/status/184098141530890240!/PaulGAllen/status/184101895126777857

So what is it Cameron might have found there? National Geographic has a few ideas.

Cameron may be detecting subtle signs of life—burrows or tracks or fecal piles—said DEEPSEA CHALLENGE biological oceanographer Lisa Levin, also of Scripps, who’s monitoring the expedition from afar.

If the water’s clear, she added, Cameron may be seeing jellyfish or xenophyophores—giant, single-celled, honeycomb-shaped creatures already filmed in other areas of the Mariana Trench. (See “Giant ‘Amoebas’ Found in Deepest Place on Earth.”)

“If we get lucky,” Cameron said before the dive, “we should find something like a cold seep, where we might find tube worms.” Cold seeps are regions of the ocean floor somewhat like hydrothermal vents (video) that ooze fluid chemicals at the same temperature as the surrounding water.

Earlier this month, during a test dive near Papua New Guinea, Cameron brought back enormous shrimplike creatures from five miles (eight kilometers) down. At 7 inches (17 centimeters) long, the animals are “the largest amphipods ever seen at that kind of depth,” chief scientist Bartlett said. “And we saw one on camera that was perhaps twice that size.”

At Challenger Deep depths, though, the calcium animals need to form shells dissolves quickly. It’s unlikely—though not impossible—that Cameron is finding shelled creatures, but if he does, the discovery would be a scientific jaw-dropper.

Even if he uncovers “a rock with a shell limpet or some kind of bivalve in the mud”—such as a clam, perhaps—”that would be exciting,” Scripps’s Levin said.

We’ll have to wait a bit to learn what information Cameron has brought back, but based on how much information we were given today as the dive progressed, it shouldn’t take long for the good stuff to get out into public view.