Everything You Wanted to Know About the Deepest Part of the Ocean

Deepest part of the ocean - Mariana trench
The Challenger Deep, located in the southern part of the Mariana Trench, at a depth of about 10.911 km (approximately 36,069 ± 131 feet), is the deepest point of the ocean floor. It is a slot-like depression in the trench, located in the western Pacific Ocean, near the Mariana islands. The exact coordinates of the point, are 11.3733° N, 142.5917° E.
Cameron's been there, done that.
On March 26, 2012, National Geographic explorer and Oscar-winning filmmaker, James Cameron, reached Challenger Deep, in the 1,580 mile long Mariana Trench, at a depth of 10,898.4 meters, in the Deepsea Challenger submersible. This is the only solo descent into the trench.
The Deepest Point
deepest point
The Challenger Deep is the lowest known point of the Earth's crust. The temperature at its depth, ranges from 1° to 4° C. With 7 miles of water pressing down on it, the pressure at the point is 15,750 psi, which is about 1000 times the standard atmospheric pressure, at the sea level. The trench is a part of a global network of troughs in the Earth's crust, that arise due to tectonic activity (sliding of the crust plate fragments).
The trench was formed from collision of two tectonic plates, which were parts of the oceanic crust. As illustrated in the diagram presented below, one plate subducted (slid below), while the other climbed over, creating the deep trench. The trench is part of the Izu-Bonin-Mariana subduction zone, where the Pacific plate has dived below the Philippine or Mariana plate, creating a trench, in the process.
The Challenger deep gets its name from the British Navy vessel called 'HMS Challenger', which first made a measurement of its depth, in 1875. If Mount Everest is placed in the depths of the Mariana trench, it would be totally submerged, with about a mile of water above it.
challenger deep comparison
In 1960, humans reached this ultimate depth, through a special underwater exploration craft, called 'Bathyscaphe Trieste'. The submersible vessel reached the deep on 23rd January, 1960. At that depth, the two men (Jacques Piccard and Lt. Don Walsh), who were operating the submersible, could do little except measuring the depth, as their descent raised debris, which reduced their visibility to almost zero.

Later in 1996, the Japanese remotely operated underwater vehicle (ROV), Kaiko, made the deepest unmanned dive ever and brought home, some samples of bacteria, living in these depths. One of the most productive dives in the Challenger deep was made in May, 2009. The US-built hybrid ROV 'Nereus' reached the Challenger deep and spent 10 hours on the ocean floor, sending live video data and collecting biological samples, using its manipulating arm. Most of the surviving life in those depths of the Pacific ocean was found to be soft-shelled organisms.

In circa 2012, James Cameron dived in solo, in the Deepsea Challenger, to explore the Challenger Deep and spent several hours exploring the deep sea environment. As part of the mission, besides 3D camera shooting, biological samples were also taken in, for analysis. Many future missions have been planned, that will explore the depression in even more detail.
Very close to the Challenger Deep, are two more depressions in the sea floor, that are possibly the second and third most deepest points in the oceans. Both are located in the Pacific. Here are their vital statistics.

Sirena Deep a.k.a. Hawaiʻi Mapping Research Group (HMRG) Deep
12.0654° N, 144.5811° E || 35209.974 Feet/10732 m

Horizon Deep
23°15.5′ S 174°43.6′ W || 35,433 ± 33 feet/10,800 m ± 10 m
The closest piece of land to the Challenger Deep, is the Fais island, which is about 289 km southwest of it and the second nearest is Guam, which is about 306 km to the northeast.

Recently, Challenger deep was suggested as the best site for nuclear waste disposal. It being the tip of a subduction plate, the nuclear waste would be pushed into the depths of the Earth's mantle. However, it's also probable, that the waste might be pushed up by tectonic activity later. Opinions remain divided on the issue.
Deepest Points in Oceans
Arctic Ocean: Litke Deep (in Eurasian Basin) || 17,881 feet/5,449 m

Atlantic Ocean: Milwaukee Deep (in the Puerto Rico Trench) || 28,373 feet/8648 m

Indian Ocean: Sunda Trench a.k.a. Java Trench || 23,376 feet/7125 m

Southern Ocean: South Sandwich Trench (Southern End) || 23,736 feet/7235 m
Almost every one of these deeps, being parts of subduction points, are potential sites for origin of earthquakes, leading to powerful tsunamis. Ergo, monitoring them, is one of the most important tasks.

All the past missions into these depths have found microbial and multicellular life forms thriving in the conditions, that were earlier thought to be nearly impossible for life to survive. The observations are testimony to the fact that life on Earth is most resilient, tenacious, and adaptable.