The Mariana Trench in the Pacific has been identified recently by Pew as a unique biodiversity reservoirs that deserve to be protected.
It is a good positive reminder of the evolving human vision of the global environment: in the 1980s and up until the early 1990s several countries (Japan, Taiwan, the US...) were planning to use the Mariana Trench as an international radioactive wastes dumpsite. The prevailing conventional belief then was that radioactivity could be conveniently dispersed into the ocean, rather than isolated from the environment. And those of us (Greenpeace, the Pacific Island States, among others) who said "Wait a minute, doesn't what we do not know exceed what we know?" were often treated as lunatics by the nuclear powers at the time.
David Gallo's online film Underwater Astonishments provides a good snapshot of the wonders of undersea living features.