On Aprl 10 the U.S. research vessel Thomas G. Thompson will steam 900 kilometers northeast from New Zealand and stop in the wide open Pacific Ocean. If all goes according to plan, it will drop Nereus, a robotic vehicle the size of a subcompact car. Nereus will dive, and dive, and dive down to one of the deepest and most hostile places on earth: the Kermadec Trench. It will hit bottom at just beyond 10,000 meters—the extent of Mount Everest, plus a modest Smoky Mountain. There, in frigid, absolute darkness, under water pressure of 15,000 pounds per square inch—the equivalent of three SUVs pushing down on your big toe—Nereus will shine its lights on the unknown. A video camera will stream imagery back up to the Thompson along a drifting, fiber-optic filament the width of a human hair, which Nereus will have spooled out as it sank.

Scientists onboard the Thompson will be glued to their computer screens to see what strange life-forms appear. As they watch, Nereus's robot arm will grab animals and rocks from the trench floor. It will thrust a stiff tube into the seabed and pull up a core sample of the sediment there. And the robot will slurp glassfuls of water in hopes of trapping bacteria and other organisms that manage to survive in the extreme conditions.

Biologists and geologists have every reason to believe Nereus will reveal amazing wonders. But the expedition has a still greater significance. Humans have rarely ventured below 6,000 meters, to the ultradeep trenches worldwide known as the hadal zone. The April expedition, led by the Woods Hole Oceanographic Institution (WHOI), marks the beginning of an era scientists have spent decades fighting and longing for: a systematic exploration of the planet's final frontier. The Nereus mission is “the dawn of hadal science as an enterprise,” says Patricia Fryer, a marine geologist at the University of Hawaii at Manoa. “And it's an enterprise that could very well provide us with some incredible discoveries.”

Hadal exploration is ready to take off because the stars of funding, technology and publicity have aligned. The public's attention was riveted on the hadal zone in 2012, when movie director and explorer James Cameron piloted a one-man submersible down to the bottom of the deepest place on the planet, in another trench called the Mariana. WHOI has improved the deep-sea technology needed to make Nereus strong yet nimble. Funding is rising. And with other vehicles being built, extended access to the deepest places in the world is becoming realistic.

Money is still tight, of course, and the task is enormous—the trenches of the world's hadal zone occupy an area nearly the size of Australia. Where should deep-sea vehicles go? What should they look for? In interviews with more than a dozen ocean experts, the consensus converges on a small number of top priorities. Among them: Figuring out how creatures survive such crushing pressure. Investigating whether organisms big and small host novel compounds that could lead to new drugs. Determining how tsunami-spawning earthquakes are born. And answering the ultimate question: Could the trenches have spawned the start of life on earth, as some scientists have suspected but have had no way to prove or disprove?

New Creatures Galore
The Nereus mission could make strong progress on several priorities in the research agenda—if it survives the water pressure itself and if its robot arm and sensors work. The $8-million robot's greatest strengths are that it can take live video and cover far more ground and collect more rock, sediment and water samples than the many “landers” that researchers have dropped off ships in the past—the small pods that sink to one spot on the bottom, providing useful but limited information. Nereus can also stay submerged for up to 12 hours, and even if the tether breaks it can return to the ship on its own.

Those capabilities mean Nereus may be well suited for surveying bizarre life-forms—the first order of business on the agenda. Until now, scientists have been searching in isolated places. Nereus will take them on an extended virtual trip through the Kermadec Trench, collecting all kinds of biological samples as it goes. “I think we'll be surprised,” says Timothy Shank, a WHOI deep-sea biologist and chief scientist for the April mission. “It'll be things we haven't thought about, even though we think we've thought about everything. That's what's driving me.”

A visitor to the laboratory where Shank stores preserved samples of brittle stars, shrimp, tube worms and other deep-ocean dwellers, will see shelves marked with names of intriguing locations around the globe, such as the Galápagos Rift or the Mid-Atlantic Ridge. But not a single shelf sports the name of a hadal trench because so few samples have been collected.

With rare exceptions, manned and unmanned vehicles have been designed to plunge only a little past the top of the hadal zone, the 6,000-meter level. Intense pressure and other factors make operating deeper much more complicated and expensive. The full hadal depths remain virtually unexplored.

Only four vehicles have made it to the deepest spot on the planet—the 10,989-meter depression called Challenger Deep in the Mariana Trench, near Guam. Don Walsh, a U.S. Navy officer, and Swiss ocean engineer Jacques Piccard made the first trip in 1960 in Trieste, a massively fortified, blimp-shaped bathyscaphe. Nobody saw the spot again until 1995, when the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) sent down a remotely operated vehicle named Kaiko. The Nereus came next, reaching Challenger Deep in 2009. Three years later Cameron did it in person in his private DEEPSEA CHALLENGER submersible.

Most of the dives collected scientific samples, but the trips were primarily engineering tests—prove you can make it there, and you can make it anywhere. Kaiko offered great promise for long-term science, but after limited work in hadal regions it was lost at sea during a massive storm in 2003. Its main replacement can dive to only 7,000 meters. Another vehicle, Abismo, can go to 10,000 meters but has dramatically reduced capabilities and has seen little action.

Shank is hoping Nereus will soon fill a shelf marked Kermadec. The expedition is part of the Hadal Ecosystems Studies program, or HADES, funded by the National Science Foundation—the umbrella under which scientists from the U.S., U.K., New Zealand and Japan will begin tackling the hadal agenda.

Earlier expeditions focused on the flat center of the Challenger Deep, a sediment plain, because of its superlative depth, but that is not the most interesting spot scientifically. The most intriguing parts of trenches are the rocky outcrops and slopes along the sidewalls.

Researchers chose the walls of Kermadec as their first target because the waters above harbor lots of life, which in turn can mean more food drifting down toward life at the bottom. (In contrast, the Mariana Trench lies underneath a relatively unproductive sea.) Because Nereus can run for hours, investigators hope it will not only see but snare creatures for genetic analyses—as long as the animal is not too large or too quick a swimmer.

Kermadec “is meant to be a landmark for the first systematic exploration and a benchmark for future studies,” Shank says. “Then we'll go into other trenches for comparison.” And he is confident they will get that chance: “Momentum is so strong.”

Each trench could have its own unique set of life-forms, but HADES leaders are cautious about this proposition. Researchers once thought seamounts (underwater mountains) each harbored unique species, but further work revealed that that assumption was wrong, mostly because exploration had been too limited.

Thriving under Pressure
The second biological priority is almost as basic as the first: figuring out how cells in critters big and small can function under such immense pressure. Understanding the mechanisms could lead to new kinds of medical compounds for people.

The mystery goes back to Walsh and Piccard's 1960 dive. They spent 20 minutes on the bottom and reported seeing a flatfish, but they had no cameras. Biologists now question that observation.

“There's no way they saw a flatfish, absolutely no way,” says Jeffrey C. Drazen, a deep-sea fish specialist and a HADES leader. Work suggests fish simply cannot withstand such pressure. The deepest fish ever definitively observed were seen on video at about 7,700 meters. Walsh acknowledges he is not an ichthyologist but defends his sighting. “All I can say is I think I've seen a few fish. But they keep telling me I didn't see one.”

In the 1990s Paul Yancey, a biologist at Whitman College, discovered that as depth increases, fish cells have increasingly higher concentrations of trimethylamine N-oxide (TMAO)—the same chemical that makes fish stink. That pattern has held down to about 7,000 meters—the deepest Yancey and Drazen, now a collaborator, have samples for.

Exactly how TMAO might stabilize proteins against pressure is not clear, and its effectiveness may have limits. In the fish bloodstream, TMAO functions something like salt, helping to maintain the osmotic pressure that determines whether water will diffuse in or out. At about 8,000 meters, a fish's saltiness should roughly match seawater. According to the hypothesis, if a fish went much deeper, so much water would diffuse into its cells that it would not survive. Scientists cannot prove a negative outcome, but if they do not find fish at deeper hadal depths they would have a pretty strong case against fish in those regions.

That said, Yancey says he would be happy if Nereus proves him wrong. “I'd love to have that fish, to figure out what's going on.”

Based on the little information available, other creatures such as crabs and shrimp also seem limited to about 8,000 meters. But on the few past missions researchers have seen organisms such as sea cucumbers as well as crustaceans called amphipods. Microbes have been ubiquitous. Yancey thinks these life-forms could have additional protein-stabilizing chemicals, or piezolytes, which he found in amphipods that Cameron's team collected.

Biomedical researchers were already studying another compound that Yancey turned up, scyllo-inositol, as a potential treatment for Alzheimer's disease, which is one of several that involve protein-folding problems. That relevance has biologists excited about discovering potentially useful protein stabilizers in organisms that survive in the trenches.

Doug Bartlett, a microbiologist at the Scripps Institution of Oceanography, has also done initial work with hadal bacteria collected by landers and with sediment Cameron was able to grab. Bartlett will be studying bacteria in water, sediment and animal samples Nereus collects, and he hopes to eventually get samples in containers that maintain hadal pressures, so he can observe how cells survive in real time.

Carbon and Tsunamis
Because so little hadal work has been done, a single data set from Nereus or another mission could help address top priorities in multiple disciplines. That is the case with the critical question of how much carbon is raining down or sliding down into the trenches.

For ocean organisms, carbon-based molecules are food. The cascade includes all kinds of pleasantries, such as dead algae and fish as well as poop from shallower residents. Levels can decrease with depth because much of what sinks gets consumed on its way to the bottom. But trenches could also be acting like funnels that concentrate organic matter from above and from sediment that slumps down the walls. Knowing where carbon concentrates might tell biologists where to find the greatest diversity of animals, and ample food could mean more and larger animals than expected.

Geochemists are fixated on that same carbon because the oceans absorb roughly 40 to 50 percent of the carbon dioxide that humans and nature emit into the atmosphere, slowing the greenhouse effect. Researchers think a lot of the carbon may get buried on the seafloor, but they cannot say even within a factor of 10 how much carbon is in the trenches.

Nereus will collect sediment that the HADES team will analyze for carbon. They will also measure oxygen to assess biological activity there. “I've got the world's deepest oxygen sensors that I'm dying to actually get into the trench,” Drazen says.

Geologists are interested in another aspect of the trench floor that could hit the public much closer to home, literally.

The magnitude 9 Tohoku earthquake that caused the devastating 2011 tsunami and Fukushima nuclear reactor meltdown in Japan occurred in the Japan Trench. Many scientists were shocked that a quake so large was possible there, says Hiroshi Kitazato of JAMSTEC. Researchers have drilled sediment cores there from surface ships and have a few rock samples from a robot that can go to 7,000 meters.

But the trench is 1,500 meters deeper, and the epicenter was well below the seafloor.* Experts are not even sure how to survey such areas appropriately, says Gerard Fryer, a geophysicist at the Pacific Tsunami Warning Center in Honolulu. “A huge amount will be learned if we can get some eyes down there.” Deeper rock samples could improve understanding of how stress progresses in fault zones, and more sediment samples could help researchers assess evidence that the types of sediment in trenches could affect the magnitude of quakes.

The other process that comes up when geologists make their hadal wish list is serpentinization. It is key to understanding the long-term balance between how tectonic plates are built and destroyed. The destruction occurs where trenches form, when two plates butt up against each other and one slides down underneath. Much of the material melts, and serpentinization, which involves water reacting with certain rocks, is key to maintaining this part of the balance.

Or so scientists think. Rock and sediment samples from trenches, along with detailed views of surrounding areas, could go a long way to proving or disproving theories, says Daniel Lizarralde, a geologist at WHOI. Nereus could bring that information from Kermadec or its mission to Mariana later this year. “This would be the first confirmation that processes that seem logical actually take place in reality,” Patricia Fryer says.

Cradle of Life
Serpentinization may also hold part of the answer to the ultimate question of whether life on earth emerged from the deepest sea. The process releases heat, hydrogen, methane and minerals—a recipe for chemical-based, or chemosynthetic, life. In some deep-sea locations, chemical reactions supply the energy that living organisms run on, not photosynthetic energy from the sun. Some scientists reason that life might have begun at hydrothermal vents—holes in the seafloor where seawater that has cycled into the rock below reemerges, heated and loaded with chemicals and minerals. Most of us have seen the images of huge tube worms that have been found at these places.

Vents tend to be ephemeral, however, so some scientists now question whether they could have really spawned life. A newer hypothesis proposes that serpentinization in trenches could have more readily fueled the first life because it occurs across much larger areas and is sustained for much longer in geologic time. Cameron says this idea is what most compelled him during his visit to the Challenger Deep: “I felt like I was looking at the cradle of life itself.”

In fact, during Cameron's test dives before the full descent, his team dropped an instrumented lander into the Sirena Deep, a location close to Challenger Deep and only slightly shallower. The fiberglass box, about the size of a refrigerator, had water samplers, a baited trap, a video camera and other devices. On one deployment it happened to land in front of what appeared to be a stringy, white microbial mat. “It was like playing darts blindfolded and throwing a bull's-eye,” Cameron says.

Kevin Hand, a planetary scientist and astrobiologist at the nasa Jet Propulsion Laboratory who worked on the expedition, made the discovery while reviewing video footage. When the lander hit bottom, it must have kicked up some microbes, which were captured by a sampler. Early results suggest that the bacteria have genes that would enable them to use compounds released by serpentinization to create energy.

Of course, there is no reason hydrothermal vents could not also exist in the deepest trenches. The species seen at shallow vents are odd enough; add the daunting pressure of a trench, and there is no telling what kind of strange life might be found.

Underwater Space Race
The agenda for hadal scientists is exciting but for now will be difficult to accomplish, with only one primary vehicle, Nereus, at their disposal. Cameron has given WHOI rights to the technologies developed for his sub, as well as the sub itself, but the institute has no plans yet to dive with it, in part because of insurance issues.

The options could increase by the end of 2015. The Schmidt Ocean Institute, founded by Google executive chairman Eric Schmidt and his wife, Wendy Schmidt, is working with WHOI to build a Nereus successor called N11K. It will have a greater payload for samples and two robot arms instead of one so it can grab hold of the seafloor with one arm and really dig into it with the other. [Disclosure: I am a consultant for Schmidt on media communications.]

Manned vehicles are not likely to play a major role, at least for the next few years. As in the U.S. space program, debate is under way over the relative merits of human-occupied vehicles. Shank and Patricia Fryer say video cannot replace a person's 3-D visual ability to make sense of what is seen at deep, dark depths.

Cameron adds that manned missions provide inspiration [see box on opposite page]. Indeed, Virgin Group founder Richard Branson and partner Chris Welsh, an entrepreneur, intend to pilot a one-person submersible to the deepest spot in each of the planet's five loosely defined oceans. Problems with their prototype vehicle's clear dome have slowed the project.

Triton Submarines in Vero Beach, Fla., has designed a three-person sub that would reach the deepest trenches but has not acquired the funding needed to build it. China, which recently christened a 7,000-meter manned vehicle, is designing one to reach full hadal depth, as is Japan, but both projects are years from completion.

At JPL, Hand is designing a small autonomous vehicle that could be launched in groups over the side of a small vessel. He says the cost target is about $10,000 a pod, so fleets would be possible and losing one would not be catastrophic. He, along with engineers from JPL and from Cameron's group, is seeking nasa funding. “We want to take some capabilities and lessons learned from robotic exploration of worlds like Mars and beyond and bring that capability and the talent to bear on the exploration of our ocean,” Hand says.

As exploration technology and money expand, researchers will face the welcome challenge of deciding which trenches and troughs to plumb next. Very different creatures might live in the remote conditions of the 8,428-meter-deep South Sandwich Trench just above Antarctica. The Puerto Rico Trench could offer valuable information about connections between trenches. And there are a few open seafloor plains in the hadal zone that researchers would love to compare against trenches.

Patricia Fryer is trying to secure funding for a workshop that would bring together deep-sea scientists from around the world to discuss priorities and the best ways to move forward. Prior research was piecemeal, and as in outer space, coordinated international work could most efficiently exploit funding and expertise, rather than a competitive race to the bottom among nations. “I think the community of marine scientists is ready” to complete a systematic hadal agenda, Fryer says. Drazen agrees: “We have the technology now to explore these places. And people are champing at the bit, ready to go.”

*Erratum (5/27/14): The usage of the term "epicenter" in this sentence is incorrect. It should read, "But the trench is 1,500 meters deeper, and the earthquake began well below the seafloor."