The article linked to above has a great summary of the last (and only) successful dive into Challenger Deep, the deepest part of the Trench, which was the Trieste dive of 1960. The Trieste was a custom-built submersible, comprising a spherical capsule in which the two crew sat with their instruments, suspended underneath a giant container of gasoline. It functioned like an underwater blimp: the gasoline acted as the buoyant balloon, and they carried ballast to bring them down, which was jettisoned for the ascent. It's a great story of engineering and derring-do, unfortunately not much was learned about the biology of the deep sea from that expedition.
I particularly liked the writer's description of how life can survive at such high pressures in the deep:
One oceanographer described how as follows: a human is like a party balloon – taken down to great depths it will be crushed to nothing; but a deep-sea fish is like an untied balloon – take that down to the bottom of the ocean and the pressure has little effect. Perhaps the greatest difficulty is that intense pressure causes nerves to stop working, a problem that deep-sea life has side-stepped by evolving a more robust physiology at molecular level.Cameron's quest is not really a scientific expedition, but is still an exciting opportunity for observation of life in the deep sea. The director is also known for his underwater documentary film-making, such as Volcanoes of the Deep Sea featuring the submersible Alvin). Given that the marine realm has inspired many of Cameron's cinematic works (Aliens, The Abyss, Avatar, and of course Titanic) it's only natural that he returns to it for inspiration, like marine enthusiasts all over the world, except with deeper pockets!