The world's largest ecosystem? Not the Amazon rain forest, nor the Great Barrier Reef. It is the abyss.
Far below the depth that light can penetrate the oceans is the abyssal zone, which starts at 2,000 meters. The pressure is huge -- hundreds of times that at the surface -- and its volume is immense (after all, oceans cover more than two-thirds of the Earth's surface). Until 1977 it was thought that nothing lived at those depths that did not feed on material falling from higher up, where the Sun's energy powered life.
But things do inhabit the abyss -- and some are more alien than anything seen on "The X Files."