Titan sub: What is a 'catastrophic implosion'?

The Titan submersible, operated by OceanGate Expeditions to explore the wreckage of the sunken Titanic off the coast of Newfoundland, dives in an undated photograph. OceanGate Expeditions/Handout via REUTERS

BOSTON (AFP): A "catastrophic implosion", such as that believed to have destroyed the Titan submersible, would have happened with incredible force and speed given the crushing water pressure on the floor of the ocean.

The remains of the Titanic rest on the seabed in the North Atlantic at a depth of some 3,800m