A sub's Test Depth is generally set at two thirds design depth for US submarines and four sevenths design depth for the British Royal Navy, and is the maximum depth at which a submarine is allowed to operate during peacetime.
If you've got a submarine going underneath the ice, you can look upwards and it can tell you what the thickness of the ice is, because you can't do that from looking from above.
France has taken the unprecedented step of recalling its ambassadors to Australia and the United States amid the continuing row over the cancellation of a submarine contract and the creation of a new security alliance.
On one occasion, while touring a submarine for a radio programme I was making, the submariners showed me their sleeping quarters, where the bunks were stacked so closely on top of each other that it was hard to turn over.
Alas though, Unsinkable Sam's luck wasn't holding out. He had been on the Ark Royal for less than a month when it was hit by a torpedo from a submarine attack by U-boat U-81 on 14th November, 1941.
The Argentinian Navy says it believes there was an explosion in the ocean close to the last known location of a submarine which went missing off the coast of Patagonia.
As they float down a stream, the Baudelaires encounter a submarine, the Queequeg, which is captained by Captain Widdershin, a man who speaks very fast and never likes to hesitate.