After years of preparation and days of uncooperative weather, James Cameron, at approximately 3:15 p.m. ET (5:15 a.m., local time), began descending solo to Earth's deepest, and perhaps most alien, realm, according to members of the National Geographic expedition.

If all goes to plan, within two hours of his submersible's launch, the National Geographic explorer and filmmaker should become the first human to reach the Mariana Trench's Challenger Deep alone—and the only one to explore it in depth, in person.

Cameron's "vertical torpedo" of a sub, as he calls it, has already made the nearly 7-mile (11-kilometer) trip to Challenger Deep and back, unmanned and unscathed, Cameron told National Geographic News on Friday. (See pictures of Cameron's sub.)

"We did some test launches and recoveries, and we did an unpiloted dive of the vehicle," Cameron said in a phone interview Friday.

In predawn darkness Monday, local time, Cameron folded himself into the cockpit—a steel sphere as cramped as any Apollo capsule—and the hatch was literally bolted shut.

Shortly after a crane had lowered the submersible into the Pacific and support divers had detached flotation devices, he began sinking fast, and the cockpit, balmy at launch, began to cool. In the sunless depths about 13,000 feet (3,960 meters) down, bioluminescent creatures likely began flickering into view like living constellations.

At the same time, Cameron's voice began to dim as the sub's audio signal grew weaker, according to reports from a support ship.

Once at bottom, Cameron will collect data, specimens, and imagery unthinkable in 1960, when Walsh and Piccard left Challenger Deep having seen little more than the silt stirred up by their bathyscaphe.

After as long as six hours in the trench, Cameron—best known for creating fictional worlds on film (Avatar, Titanic, The Abyss)—will jettison steel weights attached to the sub and shoot back to the surface.

Meanwhile, Cameron's team will await his return aboard the research ships Mermaid Sapphire and Barakuda.

Despite the confining conditions and extreme isolation of the sub and the dive, physician Joe MacInnis said he's not worried about Cameron's health during the Mariana Trench dive. (Find out more about what Cameron will experience.)

"He's like the early Mercury astronauts. He's alone in a capsule, and now he's spent enough time in that very confined crew cabin to really be comfortable," said MacInnis, a long-time Cameron friend and a member of the DEEPSEA CHALLENGE project, a partnership with the National Geographic Society and Rolex. (The Society owns National Geographic News.)

To prepare for the Challenger Deep descent, Cameron has been running several miles daily, practicing yoga to increase his flexibility, and immersing himself in deep-ocean science, MacInnis said.

"He's got a very quick-study mind ... and he's been talking to scientists for several years," MacInnis said. "But he's really been focusing for the past year."

The preparation should be worth it, according to Andy Bowen, project manager and principal developer of the Nereus, a remotely operated vehicle (ROV) that explored Challenger Deep in 2009.

The Mariana Trench dive, Bowen said, should be "an important and bold evolutionary step forward in terms of human explorations of the oceans."

Rendezvous at Challenger Deep

Upon touchdown at Challenger Deep, Cameron's first target will be a phone booth-like unmanned "lander" dropped into the trench hours before his dive.

Using sonar, "I'm going to attempt to rendezvous with that vehicle so I can observe animals that are attracted to the chemical signature of its bait," said Cameron, also a National Geographic Society explorer-in-residence.

He'll later follow a route designed to take him through as many environments as possible, surveying not only the sediment-covered seafloor but also cliffs of interest to expedition geologists.

Though battery power and vast distances limit his contact with his science team to text messaging and sporadic voice communication, Cameron seems confident in his mission: "I'm pretty well briefed on what I'll see," he said.

Bullet to the Deep

To get to this point, Cameron and his crew have spent seven years reimagining what a submersible can be. The result is the 24-foot-tall (7-meter-tall) DEEPSEA CHALLENGER.

Engineered to sink upright and spinning, like a bullet fired straight into the Mariana Trench, the sub can descend about 500 feet (150 meters) a minute—"amazingly fast," in the words of Robert Stern, a marine geologist at the University of Texas at Dallas.

Pre-expedition estimates put the Challenger Deep descent at about 90 minutes.

By contrast, some current ROVs descend at about 40 meters (130 feet) a minute, added Stern, who isn't part of the expedition.

Bowen, of Woods Hole Oceanographic Institution, called the DEEPSEA CHALLENGER "an extremely elegant solution to the challenge of diving a human-occupied submersible to such extreme depths."

"It's been engineered to be very effective at getting from the surface to the seafloor in as quick a time as possible," Bowen said.

And that's just the idea, the DEEPSEA CHALLENGE team says: The faster Cameron gets there, the more time for science. (Read more about DEEPSEA CHALLENGE science.)

Pursuing speed and science in tandem makes the DEEPSEA CHALLENGER test dives—and the Challenger Deep dive—perhaps as unorthodox as the sub itself.

Typically "you conduct a sea trial for a vehicle, you pronounce it fit for service, and then you develop a science program around it," Cameron said. "We collapsed that together into one expedition, because [we were] fairly confident the vehicle would work—and it is."

Tower of Tech

At the bottom of the trench, the sub's custom-designed foam filling and the water pressure-resistant shape of the "pilot sphere" will help protect Cameron from the equivalent of 8 tons pressing down on every square inch (1,125 kilograms per square centimeter). (Video: how sub sphere protects Cameron.)

At his disposal will be a sediment sampler, a robotic claw, a "slurp gun" for sucking up small sea creatures for study at the surface, and temperature, salinity, and pressure gauges.

While that might sound like a gearhead's paradise, Cameron knows he'll "have to be able to prioritize."

"Is my manipulator working properly? Do I still have room in my sample drawer? And do I still have the ability to take a [sediment] core sample? ... I only have [tools for] three sediment cores available on the vehicle, so I have to choose wisely when to use them."

By contrast, the sub's multiple 3-D cameras will be whirring almost continually, and not just for the benefit of future audiences of planned documentaries.

"There is scientific value in getting stereo images," Cameron said, "because ... you can determine the scale and distance of objects from stereo pairs that you can't from 2-D images."

But "it's not just the video. The sub's lighting of deepwater scenes—mainly by an 8-foot (2.5-meter) tower of LEDs—is "so, so beautiful," said Doug Bartlett, a marine biologist at the Scripps Institution of Oceanography in San Diego, California.

"It's unlike anything that you'll have seen from other subs or other remotely operated vehicles," said Bartlett, who is also the chief scientist for the DEEPSEA CHALLENGE project.

The Search for Life

It's a mystery what Cameron will see, sample, and film at depth, in part because so little is known about the Challenger Deep environment. The only glimpses scientists have had of the region, via two ROV missions, showed a seafloor covered in light gray, silky mud.

Cameron may detect 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 could see 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, "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 needed for animals to form shells dissolves quickly. It's unlikely—though not impossible—that Cameron will find 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.

Aliens of the Abyss

Expedition astrobiologist Kevin Hand, of NASA, imagines that the life-forms Cameron might encounter could help fine-tune the search for extraterrestrial life.

For instance, scientists think Jupiter's moon Europa could harbor a global ocean beneath its thick shell of ice—an ocean that, like Challenger Deep, would be lightless, near freezing, and home to areas of intense pressure. (See "Could Jupiter Moon Harbor Fish-Size Life?")

And for UT Dallas's Stern, DEEPSEA CHALLENGER's rock-sampling capability offers the opportunity to better understand our planet's inner workings.

"Challenger Deep is the deepest cut into the solid Earth," Stern said, "and this gives us a chance to see deeper into the Earth than anywhere else."

Once the trench-dive data, specimens, and imagery have been analyzed, National Geographic magazine plans to reveal the full results in a special issue on next-generation exploration in January 2013.

"A Turning Point"

By returning humans to the so-called hadal zone—the ocean's deepest level, below 20,000 feet (6,000 meters)—the Challenger Deep expedition may represent a renaissance in deep-sea exploration.

While ROVs are much less expensive than manned subs, "the critical thing is to be able to take the human mind down into that environment," expedition member Patricia Fryer said, "to be able to turn your head and look around to see what the relationships are between organisms in a community and to see how they're behaving—to turn off all the lights and just sit there and watch and not frighten the animals, so that they behave normally.

"That is almost impossible to do with an ROV," said Fryer, a marine geologist at the Hawai'i Institute of Geophysics & Planetology.

In fact, Cameron is so confident in his star vehicle, he started mulling sequels even before the trench dive.

Phase two might include adding a thin fiber-optic tether to the ship, which "would allow science observers at the surface to see the images in real time," he said. "And phase three might be taking this vehicle and creating a second-generation vehicle."

DEEPSEA CHALLENGE, then, may be anything but a one-hit wonder. To Bartlett, the Mariana Trench expedition could "represent a turning point in how we approach ocean science.

"I absolutely think that what you're seeing is the start of a program, not just one grand expedition."

Rachael Jackson of National Geographic Channels International contributed reporting to this story.

Corrected at 4:24 p.m. ET, March 25, 2012: Original version stated that the descent began at approximately 2 p.m. ET.