The Mariana Trench – The Mystery Below

The Mariana Trench, (the deepest part of the world’s oceans), is seven miles below the surface. Seven miles is a long way down…more than a mile deeper than Mt. Everest is high.

In 1875, a trench in the sea bed was discovered, in the western Pacific Ocean, near the Mariana Islands, by the HMS Challenger, using recently invented sounding equipment, during a global circumnavigation. The trench is part of a system which forms the boundary between two tectonic plates. It is about 2,550 km (1,580 miles) long, but has an average width of only 69 km (43 miles). Its maximum known depth is 10,994 m (6.831 miles), at a small slot-shaped valley in its floor at is southern end, known as the Challenger Deep; the second deepest place being the Sirena Deep, 200km to the east. At the bottom of the trench the water above exerts a pressure of 1,086 bars (15,750psi), which is over 1000 times the standard atmospheric pressure at sea level and is the equivalent of having 50 jumbo jets piled on top of you. The temperature at the bottom is 1 to 4 degrees C and the trench is cloaked in perpetual darkness. It contains active mud volcanoes and vents which bubble up liquid sulphur and carbon dioxide; small wonder that this is a little discovered area.

In 1951 the trench was sounded again by HMS Challenger11.


Early measurements were taken using precision depth gauges and multi beam echo sounders, but the first manned descent was by Don Walsh and Jacques Piccard, in 1960, in the Swiss-designed. Italian-built bathyscaphe Trieste, owned by the US Navy. The decent took 5 hours and the pair could only spend a scant 20 minutes at the bottom and were not able to take any photographs due to the clouds of silt they had stirred up. Until then scientists were dubious whether life could exist under such extreme pressure, but Trieste’s floodlight illuminated a creature Piccard thought was a flatfish, answering a question that biologists had asked for decades; “Could life exist in the greatest depths of the ocean?”

This was followed by the unmanned ROVs Kaiko, in 1996 which gathered samples and useful data and Nereus, in 2009, which remained on the seabed for nearly 10 hours. A fourth descent was made by the Canadian film director and deep sea explorer, James Cameron in 2012, in the submersible vessel Deepsea Challenger. He briefly reached 10,898m (35,756ft), but he could have gone deeper.

The 24 foot submersible resembled a long, green torpedo that moved vertically through the water. Its thrusters enabled Cameron to take a look around the ocean floor. “It’s very lunar”, he said. “You don’t expect a profusion of life”. The vessel had robotic arms, allowing him to collect samples of rocks and soils and a team of researches were working alongside Cameron to identify any new species. He found 68 new species but most of them were bacteria, also some small invertebrates. By the time he reached the seafloor several pieces of equipment had fallen prey to the immense pressure, accenting the hostility and danger of the place.

 In 2014 a high-resolution seafloor mapping survey published by researches from the University of New Hampshire said that the Challenger Deep bottoms out at 10,984m (36,037ft).

Some scientists question whether manned exploration is the best platform for this sort of research. They believe that remote or autonomous systems can collect a far greater volume of useful scientific data more economically. There are more projects in the pipeline, both manned and unmanned, so the future is exciting. These include Triton Submarines whose subs will carry a crew of 3; Virgin Oceanic sponsored by Richard Branson’s Virgin Group, which will carry a solo pilot; and Doer Marine, a marine technology company, who are planning to take a crew of 2 or 3.

Life in the Trench

The Trieste expedition lay to rest any doubts that life could exist in the Mariana Trench, but scientists still know very little about the types of organisms that live there. It is thought that the pressure is so great that calcium can’t exist except in solution, so the bones of vertebrates would quite literally dissolve; but scientists have been proved wrong many times. Food in the trench is extremely limited; dead plankton sinking from the surface must drop thousands of feet to reach the bottom. Some microbes rely on chemicals such as methane and sulphur, while others feed on marine life lower in the food chain.

Here are some of the most interesting results so far:-

  • The 1960 expedition observed what they thought was a flatfish about 30cm long. Many marine biologists are sceptical and suggest that the creature was a sea cucumber. It is possible that it was a holothurian, a new species of bizarre, translucent sea cucumber. The expedition also saw what looked like shrimp. These were probably amphipods, shiny, shrimp like scavengers.
  • In 1996, Kaiko collected mud samples containing tiny organisms.
  • In 2011 an unmanned expedition found some giant single-celled amoebas, more than 10cm in size, belonging to the class xenophyophores, which are known to act as hosts for a variety of organisms. They eat by surrounding and absorbing their food.
  • During Cameron’s 2012 expedition scientists spotted microbial mats in the Sirena Deep. These are clumps of microbes which feed on hydrogen and methane released by chemical reactions between seawater and rocks.
  • In December 2014 a new species of snailfish was discovered at a depth of 8,145m, breaking the previous record for the deepest living fish seen on video. The fish had wing-like fins and an eel-like tail.
  • Several other new species were also filmed including huge crustaceans known as supergiants.

Scientific Interest

As well as the excitement of discovering new species, scientists are particularly interested in micro organisms living in these deep trenches, which they say could lead to breakthroughs in biomedicine and biotechnology. The microscopic inhabitants might even shed light on the question of emergence of life on earth. One theory is that the serpentine mud volcanoes in the trench might have provided the right conditions for our planet’s first life-forms., and studying the rocks from the trench could lead to a better understanding of the earthquakes that create the devastating tsunamis which occur around the Pacific Rim.

The Future

There is no doubt that there is still much to be discovered in the deepest parts of the ocean and the outlook for future expeditions is extremely exciting……unless………..

…….Like other oceanic trenches the Mariana Trench has been proposed as a site for nuclear waste disposal in the hope that tectonic plate subduction occurring at the site might eventually push the nuclear waste deep into the Earth’s mantle. Fortunately ocean dumping of nuclear waste is prohibited by international law and plate subduction zones are associated with very large megathrust earthquakes….so - not a good area to dump nuclear waste!

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