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. Scroll to see the ocea