Icy cold, pitch black and with crushing pressures - the deepest part of the ocean is one of the most hostile places on the planet. Only three explorers have made the epic journey there: 11km (seven miles) down to the bottom of the Pacific Ocean's Mariana Trench. As a new wave of deep-sea exploration begins, take a look at the mysterious world that they will be plunging into.

Don Walsh was in the first sub to reach the ocean's darkest depth.

Challenger Deep is the deepest place on Earth. Don Walsh was in the first sub to reach the ocean's darkest depth.

Pressure Test: Diving 100m down With the help of some polystyrene heads, science reporter Rebecca Morelle finds out what happens at the pressure experienced 100m below sea level.

Pressure Test: Diving 1,000m down The BBC's Rebecca Morelle reveals what happens to a polystyrene head as it is subjected to the pressure 1,000m below the waves.

Atmospheric diving suit They may look as clunky as a suit of armour but some atmospheric diving suits (ADS) can enable people to reach depths of up to 600-700m. This ADS used by the French navy can reach 250m.

Elephant seal - expert diver This southern elephant seal is a fantastic diver and reach depths of up to 2,000m. Large males can weigh as much as four tonnes and live mainly on a diet of fish and squid.

Anglerfish, predator in the deep This fearsome predator has a lure above its mouth that flickers with bioluminescent light. It has evolved to catch the attention of prey and draw them near. If they come close enough, the anglerfish will strike.

Alvin - deep diving sub The long-serving, US-submersible Alvin is capable of taking up to three people to depths of 4,500m - but an upgrade is under way that will allow it to dive even deeper. Launched in the 1960s Alvin surveyed the wreck of the RMS Titanic and many others.

Scavengers that live on the edge of a trench Alan Jamieson, from Oceanlab at the University of Aberdeen, describes the animals that live at the top of an ocean trench.

Why 6,000m is a strange zone for life At 6,000m down, the trenches are full of surprises, as Alan Jamieson, from Oceanlab at the University of Aberdeen, explains.

Are snailfish the world's deepest fish? Alan Jamieson, from Oceanlab at the University of Aberdeen, says a species called the snailfish is the deepest fish he has seen in the ocean trenches.

HMS Challenger - survey vessel The British survey vessel HMS Challenger was the first to sound the depths of the Mariana Trench in 1875. The deepest point in the trench, Challenger Deep, was eventually named after her as was the ill-fated space shuttle.

Take a fly-through the Mariana Trench This fly through, created by Jim Gardner from the US Center for Coastal and Ocean Mapping, is the most detailed map of the Mariana Trench to date.

Prawn-like creatures dominate the deepest ocean In the deepest ocean, scavengers called amphipods dominate, as Alan Jamieson, from Oceanlab at the University of Aberdeen, explains.

Pressure Test: Diving 10,000m down The BBC's Rebecca Morelle reveals what happens to a polystyrene head as is taken to the pressure experienced 10,000m below the waves.

Meet the man who survived the deepest ocean In 1960, Don Walsh, with Swiss engineer Jacques Piccard, made the world's first dive to the bottom of the Mariana Trench. Here, he tells the remarkable story.