How deeply can a submarine with a crew safely submerge?

With enough water and ballast on board, a submarine can submerge straight to the bottom of the ocean if you're not careful.

There are 3 specific depths that submarine crews maintain constant awareness of:

1. Test Depth

2. Crush Depth

3. Bottom Depth

Of course, it is extremely important that you don't hit #3 before #1 and #2.

Test Depth is the depth at which submarines submerge to in order to test repairs, maintenance on hull valves, and to verify hull integrity. It's also used to verify a boat's readiness for operations in preparation for a deployment. TD is generally a few hundred feet above Crush Depth, which is the structural failure point of the hull at which sea pressure will overcome the hull's integrity.

The actual TD and CD's of submarines are classified - even published figures are only estimates. For example, during the Cold War the Russian ALFA class SSN was estimated to have a crush depth of nearly 3000' - we know today that her TD was only around 1200', putting her CD a lot shallower than 3000'. Official acknowledgment by the U.S. Navy is greater than 400' for all U.S. submarines.

One of the arguments by politicians then was about how the Russians could build a submarine capable of going to such depths when NATO couldn't. The answer was as simple then, as it is today; at such depths, you don't need another submarine capable of following them that deep. You only need a torpedothat's fast enough and can dive that deep to catch them. At that depth, it doesn't take much damage to cause a major unrecoverable casualty.

The materials which a submarine are constructed with affect a boat's overall depth capability as well. U.S. and NATO submarines are typically constructed of highly flexible steel which can withstand repeated excursions to deep depths, and can contract and expand with sea pressure changes.