This article was written by Alan Jamieson from the University of Aberdeen and was originally published by The Conversation. 

The ocean is deep. In fact, most of it is deep. Officially anything deeper than just 200 metres is considered the 'deep sea', but the average depth of the entire ocean is about 3.5 km and the deepest point – the Challenger Deep in the Mariana Trench, in the western Pacific - is a little short of 11 km down. That means that most of the living space on Earth is in the deep sea.

We scientists like to categorise things and the ocean depths are no exception. Depths from the surface to 0.2 km is known as the 'littoral zone', from 0.2 km to 3 km, the 'bathyal zone', and from 3 km to 6 km, the 'abyssal zone'. Anything deeper than that is the 'hadal zone'.

The hadal zone is largely comprised of deep trenches caused by tectonic plate subduction that drive the vast abyssal plains steeply down to depths of 11,000 metres in places. But even here, animals thrive, blissfully unaware of how little attention they receive. Here's an insight into their incredible world.

1. The kingdom of Hades

The term hadal comes from Hades, which refers both to the Greek kingdom of the Underworld and the god of the Underworld himself, Hades (brother of Zeus and Poseidon). The term can also mean the "abode of the dead".

In modern times, Hades is seen as evil, but in mythology he was often portrayed as unreasonably stringent rather than actively malicious. Interestingly, he strictly prohibited the inhabitants of his dominion to leave, which is a rather apt analogy for hadal fauna, as these species are often confined to trenches and are rarely capable of going elsewhere.

Cusk eel at 6,500 metres. Alan Jamieson

2. Exactly how deep is the ocean?

The extreme depths of the hadal trenches were discovered using 'bomb sounding', whereby someone threw a half-pound block of TNT off a ship and the echo was recorded on board the ship.

This method was used to sound the depths of many trenches, but the exact depth of the deepest point, currently in the Mariana Trench, is still difficult to compute. Four other trenches, all in the Western Pacific, also exceed 10 km: the Tonga, Kuril-Kamchatka, Philippine, and Kermadec trenches.

3. Who has explored it?

The HMS Challenger expedition (1873 to 1876) was the first to sample hadal depths – having collected sediment from about 8 km – although it could not confirm whether or not the sediment was merely the remnants of shallower animals. The 1901 Princess Alice expedition successfully trawled specimens from over 6 km.

However, it was a 1948 Swedish expedition, which successfully trawled a variety of species from 7 km to 8 km in the Puerto Rico Trench, that finally proved that life existed at depths greater than 6 km. In 1956, the first photographs of the hadal zone were taken by none other than Jacques Cousteau in the Romanche Trough in the Atlantic.

4. How big is the hadal zone?

The hadal zone comprises a series of disjointed trenches and other deep spots. There are 33 trenches and 13 troughs around the world - 46 individual hadal habitats in total. The mean depth of the trenches is 8.216 km. The total area of the hadal zone is less than 0.2 percent of the entire seafloor but accounts for 45 percent of the total depth range. It is therefore surprising that the deepest 45 percent of the sea is rarely mentioned in deep sea literature.

A large prawn, 7,000 metres down in the Kermadec Trench. Alan Jamieson

Of the 33 hadal trenches, 26 (84 percent) are located in the Pacific, three are found in the Atlantic (8 percent), two (4 percent) in the Indian Ocean, and two (4 percent) in the Southern Ocean. The majority run up the western Pacific. Most of the hadal trenches in their modern form are believed to have formed 65.5 million years ago during the Cenozoic period.

5. The secret behind our existence

Earth appears to be the only terrestrial planet with subduction zones and plate tectonics. Both Mercury and Earth's Moon are tectonically dead. Mars appears to have tectonically ceased, and Venus is dominated by thick lithosphere with mantle plumes. On Earth, subduction zones produce continental crust, which can protrude from the ocean (the continents).

It has been speculated that without subduction, the land would still be underwater and terrestrial life, including humans, would never have evolved.

6. What's it like down there?

Bottom water temperatures are cold and vary between 1°C and 4°C. However, hydrostatic pressure increases linearly by 1 atmosphere (atm) for every 10 metres of depth. The pressure at hadal depths therefore ranges from 600 to 1,100 atmospheres. The pressure at the deepest point is, therefore, equal to a 1-tonne weight being placed on the end of your finger.

A snailfish swims 7,400 metres below the surface. Alan Jamieson

7. What lives there?

Many marine organisms are found at hadal depths and the most common groups are the polychaetes, bivalves, gastropods, amphipods, and holothurians. All of these groups are found at full-ocean depth and often in large aggregations. Contrary to popular media, the hadal zone is not a mysterious realm inhabited by aliens or "monsters of the deep".

Instead, it is a poorly understood region largely inhabited by hoppers, snails, worms, and sea cucumbers. In fact, the upper trenches are inhabited by little pink fish and bright red prawns.

8. But there are some horrors …

In the 1970s, the Puerto Rico Trench was a pharmaceutical waste disposal site. The figures are astonishing: in just five years, more than 387,000 tons (351,080 tonnes) of waste material were discarded in the trench, an amount equivalent to 880 Boeing 747s. In addition, the ill-fated Apollo 13 mission to the Moon in 1970 carried a radioisotope thermoelectric generator (RTG) that was supposed to remain on the Moon with the lunar lander.

The RTG contained 3.9 kg of plutonium-238, and in the end was jettisoned over the south-west Pacific, where it reportedly survived re-entry and settled in the Tonga Trench at a depth of 6 km to 9 km where it should now remain radioactive for several thousand years.

9. Time-shifting earthquakes

The 2011 magnitude 9.0 Tōhoku-Oki earthquake off Japan was caused by a fault rupture in the Japan Trench. The event and subsequent tsunami left about 20,000 dead or missing and affected more than 35 coastal cities. The quake was followed by 666 aftershocks that exceeded magnitude 5.0. The energy involved in high-magnitude earthquakes originating in trenches is immense.

The 2004 Sumatra-Andaman earthquake in the Java Trench caused a sufficiently massive release of energy to alter Earth's rotation, shortening the day by 2.68 microseconds. Similarly, the Tōhoku-Oki earthquake shifted Earth's axis by between 10 cm and 25 cm, shortening the day by another 1.8 microseconds.

10. How deep is deep?

One of the most common analogies used in trench science is "Mount Everest would fit into the Mariana Trench with a mile or so to spare". This is true, and from an evolution and physiology perspective it is immense. Likewise, exploring these extreme depths is highly problematic.

But how far is 11 km really? The Mississippi River is 11 km at its widest point, Manhattan Island is twice as long as the Mariana Trench is deep, and assuming the average running speed of Mo Farah at the 2012 Olympics, he could run 11 km in 30 minutes. Given how easily we can affect our planet over far larger distances, our effective proximity to these extreme locations means even the deepest places on Earth are no longer pristine - and remain hugely vulnerable.

Alan Jamieson, Senior Lecturer, Oceanlab, University of Aberdeen.

This article was originally published by The Conversation. Read the original article.