The Test Depth is a value assigned to a submersible (usually a serving submarine as opposed to an unpowered bathyscaphe) which indicates the deepest that the submarine has been taken to (and remained at for some period of time) in its initial sea trials. The understanding is that this is the 'never exceed unless it's an emergency' depth rating, akin to aircraft V-speeds; while this depth may be exceeded - and is, from what I'm told by older submariners - it is only done so in the knowledge that the boat has not been officially proven to remain sound below that point.

Anecdote: One submariner I know (who served in the Cold War U.S. Navy's nuclear submarine force) said that among the varying slang terms for this rating, his favorite was "balls deep." Apparently, this referred to the depth past which a sailor's scrotum began to contract out of nervous tension. A more refined version was "Crush Depth," which described the effects of venturing too far down while maintaining a sealed hull.