A ship full of marine scientists is floating over the deepest part of the world: the Pacific Ocean's Mariana Trench. They're sending down probes to study life in one of the most hostile environments on the planet. This week the researchers are targeting the two deepest spots in the trench — the Sirena Deep and the Challenger Deep — which each extend down about 7 miles beneath the ocean's surface. Douglas Bartlett, from Scripps Institution of Oceanography, UC San Diego, is the chief scientist aboard the research vessel Falkor, which is operated by the Schmidt Ocean Institute. Bartlett describes undersea trenches like the Mariana, which stretches hundreds of miles across the floor of the western Pacific, as "inverted islands of biodiversity."