The deep-sea is an undiscovered world. We have explored the highest mountains and deepest valleys on land but the deepest portion of the ocean, the Mariana Trench (Pacific Ocean), has only been explored a few times, once in 1960 the US Navy dived almost 11 km beneath the surface using the 'Trieste', and a couple of times in the 1990s using the remotely operated vehicle 'KAIKO 7000II'. In 2009 Woods Hole Oceanographic Institution dived their robotic submarine Nereus to an amazing 10,902 m in the Challenger Deep, the deepest region of the Mariana Trench.

To this day, the 1960 Trieste dives are the deepest people have ever been within the oceans - the largest environment on Planet Earth.

Click here for a fly-through animation of the Mariana Trench

Who knows what other wonders are hidden in the deep-sea? Scientists have made a start but have investigated less than 4% of the deep seabed. Only as recently as 1977 did scientists discover hydrothermal vents with their unique communities of marine life. Many of the large cold-water coral reefs featured on this site were only discovered in the last 10 or 15 years.

The deep-ocean is often thought of as a stable environment, characterised by cooler temperatures than the surface waters. Sunlight is quickly absorbed by seawater, with very little light penetrating beyond the first 200 m. Plants cannot grow without sunlight, so plant life is restricted to maximum depths of approximately 50-100 m.