Scientists have identified a region of the sea floor with a depth that rivals the Challenger Deep which, at about 11,000 metres (36,000 feet), is the lowest spot on Earth.
By Dr David Whitehouse
BBC News Online science editor
The new location lies 200 kilometres (124 miles) further to the east, along the Marianas Trench in the western Pacific.
It was found during a sonar sea-floor mapping project carried out in 1997 and 2001 by scientists from Hawaii, US.
The Challenger Deep has been visited by a number of manned and unmanned submersibles since its discovery, but at present there is no craft in operation that can survive the crushing pressure at its bottom.
Sea floor mapping project
It was during a detailed survey near the island of Guam that scientists from the Hawaii Institute of Geophysics and Planetology found the new super-deep spot.
They towed a submersible containing a sonar mapping system behind a research ship.
Mapping the sea floor by sonar
This produced data on the sea floor that identified previously undiscovered faults, landslides and so-called mud volcanoes.
The scientists have named the new low point HMRG (Hawaii Mapping Research Group) Deep.
Exact measurements of its depth are difficult to make but, the researchers say, it must be close to that of the Challenger Deep.
The Challenger Deep was discovered in 1951 by the Royal Navy ship Challenger 2, after which it is named.
It has been visited by men only once, in 1960, when two scientists made a four-hour descent in the dark in a submersible, touching the bottom at 10,915 metres (35,813 feet).
The location is so deep that Mt Everest could be placed within it and still have 1.6 km (1 mile) of water above it.
Out of reach
No submersible currently in use can reach either the Challenger or the HMRG Deep.
The only recent vessel that could, the Japanese unmanned Kaiko which reached the bottom of the Challenger Deep in 1995, was lost in May after it broke its tethers during a storm.
The few submersibles that have gone to the deepest spot on Earth report that it is not lifeless. Lugworms, skeleton shrimps and nearly 200 types of microbes have been detected there.
Kaiko goes deep: Its technology is no longer available
The deepest region of the Pacific Ocean, the Marianas Trench lies at the boundary between two of the Earth's tectonic plates - the eastern edge of the Philippine Sea plate and the Pacific plate.
As they shift their positions across the Earth's surface, the Pacific plate is forced to dive beneath the Philippine Sea plate in a process called subduction.
It is this great down movement of rock that gives the Marianas Trench its huge depth.