On 26 March 2012, film director James Cameron was hunched in the cramped cabin of a submersible, far out in the Pacific ocean. His vessel, the Deepsea Challenger, looked a bit like a lime-green cigar. Over the course of two and a half hours, Cameron piloted the submersible down to a depth of 10,898 m, setting a new world record for a solo descent. He had reached the deepest part of the ocean, the Mariana Trench.

Cameron was only the third person to look upon the desolate, almost lunar landscape at the bottom of the trench. Fortunately for the rest of us, he took 3D Hi-Definition cameras with him on the dive. The films he captured show that remarkable sea life exists all the way to the bottom. They are being pored over by scientists around the world, in a bid to figure out what it takes to live in the ocean's depths.

Cameron's dive is the latest step in a 200-year journey to the deepest depths of the sea, the last unexplored frontier on Earth. At the bottom of trenches like the Mariana, the water is freezing cold, there is no light, and the pressure is pulverising. Yet somehow, life endures, and we are only just beginning to learn how it does so.

Until the late 1800s, little was known about the oceans. Folklores and myths conjured up images of terrifying sea monsters like the Norwegian kraken, and the science fiction author Jules Verne imagined that the heart of the ocean could contain "huge specimens of life from another age". But most scientists thought the darkness and cold would make the deep sea uninhabitable.

The deeper he looked, the fewer organisms he discovered

The idea that the deeps were a muddy desert devoid of life had been around for centuries. The Greek philosopher Socrates, who lived 2400 years ago, apparently said: "Everything is corroded by the brine, and there is no vegetation worth mentioning, and scarcely any degree of perfect formation, but only caverns and sand and measureless mud, and tracts of slime."

In the Victorian era, this idea was championed by a scientist called Edward Forbes. He dredged the Aegean Sea and found that the deeper he looked, the fewer organisms he discovered. He concluded that life ceased to exist beyond 550 m.

In a book published in 1859, the year Darwin published On the Origin of Species, Forbes wrote: "As we descend deeper and deeper in this region, its inhabitants become more and more modified, and fewer and fewer, indicating our approach towards an abyss where life is either extinguished, or exhibits but a few sparks to mark its lingering presence." He was confident, but five years later Forbes' ideas were blown out of the water.

Father-and-son naturalists Michael and Georg Ossian Sars spent years dredging Norway's fjords. Then in 1864 they brought the first living sea lilies to shore, from 10,000 ft (3000m) down. Sea lilies look like flowers, hence the name, but are actually animals. Each has a stalk that attaches it to the sea bottom, and many feather-like arms that guide small particles of food into their mouths.

There was just beginning to appear a rich and in many respects peculiar deep sea Fauna

The discovery of the sea lilies sent a wave of excitement through the scientific community. They were thought to have gone extinct millions of years ago, and looked like remnants of a lost, ancient world.

They also showed that the deeps were inhabited. Many years later, Georg wrote: "So far was I from observing any sign of diminished intensity in this animal life at increased depths, that it seemed, on the contrary, as if there was just beginning to appear a rich and in many respects peculiar deep sea Fauna, of which only a very incomplete notion had previously existed."

The idea that ancient life forms had endured at the bottom of the sea was too tempting to ignore.

British scientists now took the lead. Britain's Empire wanted to rule the waves, so it needed to know what was beneath them.

Between 1872 and 1876, the British ship HMS Challenger sailed for 127,653 km in a bid to catalogue the life in all the Earth's oceans and seas. It was a journey into the unknown, just like the Apollo Moon landings in the 20th century. In total 4,700 new species of marine animals were discovered, including the first records of deep-sea organisms.

The expedition discovered the ocean's deepest point

The animals discovered were extraordinary. The chief scientist Sir Charles Wyville Thomson described a hydroid, a stalk-shaped animal with tentacles, found in the north Pacific. It was "a giant of its order, with a stem upwards of 7 feet high, and a head nearly a foot across the crown of expanded tentacles."

But just as important, the Challenger team studied the landscape of the ocean floor. Thomson described "gently undulating plains, extending for over a hundred millions of square miles at a depth of 2500 fathoms beneath the surface of the sea, and presenting like the land their local areas of secular elevation or depression, and centres of more active volcanic disturbance."

The expedition also discovered the ocean's deepest point. South of Japan, there is a crescent-shaped canyon called the Mariana Trench, and at its southern end the sea bed seemingly falls away. In 1875 Thomson and his team recorded a depth of 4,475 fathoms (8,184 m). Now we know it goes at least 10,916m down. This near-bottomless pit is called the Challenger Deep, after the ship.

Almost a century later, humans went to the bottom of the Challenger Deep. On 23 January 1960, oceanographer Jacques Piccard and Lieutenant Don Walsh of the US Navy piloted a submersible called Trieste into the Deep.

It now seems the deeps are teeming with life

The descent took four hours and 47 minutes. Most of the ship was taken up with floats and water ballast tanks, so Piccard and Walsh found themselves in a 7-ft-wide spherical cabin attached to the underside, with a small Plexiglas window. They were, to put it mildly, cramped. Once they reached the bottom, they couldn't take any photographs due to the disturbed silt.

Regardless, the debate about whether life could exist at the bottom was settled. The Trieste's floodlights illuminated a creature that Piccard thought was a flatfish, but is now thought to have been a sea cucumber. "Here, in an instant, was the answer," Piccard wrote in a book about his journey.

It now seems the deeps are teeming with life. In late 2014, Jeffrey Drazen of the University of Hawaiʻi at Mānoa in Honolulu led an expedition to the Mariana Trench. His team used five remotely-operated vehicles to explore the deepest parts of the trench, as well as along the trench walls. He was surprised by the amount and diversity of life on the walls of the trench.

The first challenge animals encounter as they move deeper is the complete darkness. Some deep-sea fishes, like the stout blacksmelt, have giant eyes to capture the faintest glimmers. Others have abandoned vision. The tripodfish, named for its elongated fins that allow it to perch on the sea floor, relies on touch and vibrations to sense its prey. Still others emit their own light by a process known as bioluminescence. These lights can be used as headlights, as in lanternfish, or to attract mates or prey.

Occasionally the scavengers are treated to a dead whale

The darkness also causes a second problem. Lack of sunlight means no algae or plants to support the food chain, so food is scarce. Deep-sea animals must survive on the decaying scraps of dead organisms from the upper layers of the ocean, which sink to the bottom. A rich supply of this stuff might account for the rich ecosystem Drazen found in the Mariana Trench. "It is possible that the trenches are funnelling detritus food down," he says.

Occasionally the scavengers are treated to a dead whale, which provides an enormous feast. Hagfish burrow into such carcasses and eat them from the inside out, while boneworms eat the bones themselves. And there are hunters, too: the ping-pong tree sponge uses sharp spikes to impale its prey.

But it's the physical properties of the deep sea that make it lethal. It is frigid: in most places, temperatures are between -1 and 4⁰C. Worse, the pressure is a crushing eight tonnes per square inch, about a thousand times the standard atmospheric pressure at sea level. It's like being crushed to death in a freezer.

This combination of pressure and cold has strange effects on animals' bodies. All animal cells are surrounded by fatty membranes, which must stay liquid to transmit nerve signals and shuttle materials in and out of cells. But under these conditions, they would solidify.

"The extreme cold and high pressures of the ocean trenches would make the fat in your cell membranes solid, just like butter in a refrigerator," says Drazen. So deep-sea animals must adapt their membranes to keep them liquid. They do this by having lots of unsaturated fats – the group of chemicals that includes vegetable oil - in their membranes. These remain liquid at low temperatures and keep the membranes loose.

It's not just cell membranes. Pressure also has a crippling effect on proteins, the huge molecules that do much of the work in our cells, such as breaking down food for energy. To function, proteins must be free to change their size and shape, for instance becoming larger. This is difficult under pressure, says Drazen. "A simple analogy is blowing up a balloon. It's easy in air, but try doing it at the bottom of a swimming pool."

Fishy chemicals

To keep their proteins from conking out, deep-sea animals collect small organic molecules called piezolytes in their cells. These piezolytes bind tightly to water molecules, which gives the proteins more space and stops water being forced into the proteins' interiors and distorting them. The deeper an animal lives, the more piezolytes they tend to have in their cells. One piezolyte, TMAO, gives fish their "fishy" smell. TMAO increases with depth, so deep-sea fish taste fishier than shallow fish.

Drazen's expedition discovered the world's deepest fish living 8145 m down

But there's a limit to this. As animals take in more piezolytes, their cells become saltier. Around 8200m down, Drazen has calculated, the cells would be as salty as the surrounding water. Any more piezolytes and seawater would rush into their cells, bursting them.

In line with this, Drazen's 2014 expedition discovered the world's deepest fish living 8145 m down. The new record-holder is a snailfish with a bulbous head and partly transparent body. Drazen believes that this is about as deep as any fish can go. If he's right, there are no fish at the bottom of the Challenger Deep.

But there's plenty more than fish in the sea. Despite the lethal conditions, James Cameron's dive revealed a plethora of animals at the bottom of the ocean. For most people, they are utterly unfamiliar.

The submersible's cameras picked up crustaceans called amphipods, which look a lot like shrimp. But whereas most of the ocean's amphipods are around 3cm long, those in the Challenger Deep were over a foot long. They are the deepest examples of "gigantism" captured in the deep ocean. A piezolyte called scyllo-inositol was found in their cells, and may help them survive the pressure.

The animals that appeared most frequently on the tapes were foraminifera: giant single-celled organisms a bit like oversized amoebas. Foraminifera are little-known, but incredibly common. They live in the sediment on sea beds throughout the world, including some thoroughly inhospitable places.

Normally, foraminifera build hard shells of calcium carbonate to protect themselves. Then they can poke out their long, sticky arms and snag food. However the intense pressures at the bottom of Challenger Deep dissolve minerals, so they can't build their shells.

Amoebas in glass houses

Instead they have adapted by building soft shells from proteins, organic polymers and even sand. Grains of sand are made from silicon dioxide, the main constituent of glass, and can withstand intense pressures. One group of foraminifera, known as xenophyophores, have taken advantage of this when building their shells. By gluing sand from ocean sediments, cast-off shells, and microbial skeletons to their own faeces, they can make pressure-proof shells. There could be 50 or even 100 species of xenophyophores in the Challenger Deep, according to Natalya Gallo of the Scripps Institute of Oceanography in La Jolla, California, who has examined Cameron's footage.

The footage also revealed what looked like a series of sticks buried in the sand. But scientists soon realised that they were sea cucumbers: worm-like creatures with leathery bodies and clusters of tentacles near their mouths. The Challenger Deep sea cucumbers had all oriented their bodies in exactly the same direction, something never before seen, possibly to ensure that they picked up as much food as possible from the currents.

But despite all these discoveries, the biggest breakthroughs might come from organisms that were invisible even to Cameron's hi-definition cameras: bacteria.

Even before Cameron went down, it was clear that the Mariana Trench was home to plenty of microorganisms. Douglas Bartlett of the Scripps Institute of Oceanography, who was in charge of the scientific side of Cameron's dive, has found clumps of bacteria attached to rocks in the Sirena Deep, east of Challenger Deep. His team is now examining them to find out how they survive.

Europa may be our best bet for finding alien life in our solar system

Their analysis of the bacteria's genes shows that they can feed off reduced sulphur and carbon dioxide. Others may feed on gases like methane and hydrogen, which are belched out of the sea bed when the tectonic plates on either side of the Mariana Trench move against one another. Such deep-sea environments are one of the main contenders for the birthplace of life on Earth. As a result, the bacteria of the Mariana Trench could help us understand how life began.

They may also help us figure out where to find life on other worlds. The conditions in the Challenger Deep are nothing like the familiar surface layers of the ocean, but they are quite comparable to Europa, one of Jupiter's moons. Europa has an icy exterior under which is thought to lie a hidden, liquid ocean with twice as much water as Earth's. Europa may be our best bet for finding alien life in our solar system, as there may be active volcanoes on the sea bed where bacteria could survive. Surely, the argument goes, if they can survive in the Challenger Deep, Europa can't be that much harder.

When James Cameron got back to the surface, he said that "in the space of one day, [he] had gone to another planet and come back". It was an apt line. His expedition and others like it will not only tell us about the extremes of life on our own planet, but may help us find life on others.