Undersea mountain is the official claim. There are still parts of the ocean unmapped. From a Yahoo! article,
outdated charts in submarine crash note the statement:
"There hasn't been a formal hydrographic sweep through that area of the ocean's bottom."
Burpee said there are currently 150 ships in the world capable of doing that kind of thorough deep water work and it would take all of them 30 years to map the world's deep water.
Guess this is just part of pushing the envelope.