Covering 70 percent of Earth's surface, the ocean boasts one of the world's most incredibly deep holes -- the Mariana Trench. Located in the Pacific Ocean just south of Guam, this deep-sea gorge was formed as the Pacific tectonic plate slid under the Philippine Sea plate. Nearly the entire trench lies below 16,400 feet (5,000 meter) of water, but the lowest known point is the Challenger Deep, a location nearly 36,000 feet (10,973 meter) below sea level [source: Whitehouse]. The British Royal Navy ship Challenger II first measured the site using echo sounding, and it is still widely considered the deepest part of the Earth's oceans.
Amazingly, the Mariana Trench has been visited by both manned and unmanned submersibles. The first and only manned descent into Challenger Deep was by Jacques Piccard and Don Walsh in the bathyscaphe Trieste, on Jan. 23, 1960. Five hours after departing the surface, the craft touched the seafloor at a staggering 35,810 feet (10,915 meters), where it encountered pressure exceeding 16,000 pounds per square inch [source: Svitil]. Since then, just two robotic vessels have descended into Challenger Deep: Kaiko, in 1995, and Nereus, in 2009. Kaiko, interestingly, captured a photograph of a sea cucumber, a worm and a shrimp during its expedition, proving that life can survive even in the crushing pressure of Mariana's deepest reaches.