The Mariana Trench was the deepest known point on Earth.
The Mariana Trench was located south of Tokyo and east of Manila. It was believed to be at least eleven kilometres deep. At the bottom, the pressure was a thousand times greater than at the surface, capable of crushing any normal surface life.
In 1940, a British naval submarine was torpedoed by the Japanese and fell into the trench. An uncrewed alien spaceship that had previously crashed there used a force field to prevent the submarine from being crushed by the pressure, and kept one of the crew, Jack Hodges, alive to use a host for the race memories of its creators. (AUDIO: Phantoms of the Deep)
In the late 1950s, twin submarines were built to investigate the the Mariana Trench; the Guernica was assembled in Spain and the Trieste in Italy. In 1959 Sam Doyle and Henry Goddard were sent down in the Guernica. The Guernica was scuttled, Sam was infected with the parasite and Henry was killed. The government covered up the incident and claimed the submarine was lost at sea.
In the 1960s, the Trieste was sent down successfully. This would be the last manned attempt in nearly 50 years.
In 2009, Sam had run out of memories and the parasite wanted a new host. The parasite made a call for help that was heard across the world. When the Torchwood Three team came to investigate, it attacked their submarine and planned to infect them next. They were able to escape the parasite using the sunk Guernica, but it was still able to infect Carlie Roberts. The parasite was killed when the Guernica rose too high, depriving it of the hydrogen sulphide in needed. (AUDIO: Submission)
In 2040 the DSV Erebus descended into the trench, and discovered the submarine that had sunk there a hundred years earlier, as well as super-intelligent squid that had also been created by the alien spacecraft. (AUDIO: Phantoms of the Deep)
John Hart remarked to the immortal Jack Harkness that he could encase him in lead and throw him into the Mariana Trench and he would still claw his way out and swim back. (AUDIO: The Death of Captain Jack)