What happens when a submarine reaches its crush depth? There are actually two crush depths. The depth at which engineers have determined it wouldn't be safe to dive below and the depth at which it actually crushes.
WW2 submarines were rated at a maximum depth, but it was impossible to know for sure just how deep they could really go. Some went well below their rated crush depth and managed surface with or without damage; some, well, they never surfaced.
At the true crush depth the outer hull finally gives in along with the fuel and air tanks and then the inner hull. Submarines have a light outer, non-watertight hull that provides a hydrodynamically efficient shape. Then there is the pressure hull which is the inner hull; it maintains the difference between outside and inside pressure.
At crush depth the sub just crumples like a tin can. As a sub approaches its crush depth the crew would hear piping and fittings giving way then the ship’s hull creaking and groaning until it finally implodes, killing everyone in a matter of seconds. The water doesn't leak in, it pours in and the air inside either forms a bubble at either end of the boat or tremendous heat is generated as the water rushes in and the boat implodes killing everyone instantly. Debris, including body parts, and fuel oil float to the surface. Very terrifying.
WW2 German U-boats generally had crush depths in the range of 660 to 920 feet. Modern nuclear attack submarines are estimated to have a test depth of 1,600 ft which means the estimated depth at which they would actually implode is about 2,400 ft. However, there are some specialized military subs that can go as deep as 4,200 ft.
Submarines had many openings in their hulls to accommodate torpedo tubes, diesel engine exhausts and air intakes, periscope, the prop shaft, etc. A submerged submarine is very delicately balanced as to buoyancy and so it takes very little flooding to overwhelm it. WW2 subs also had very little power available on their electric motors for running underwater. That meant that in the event of flooding they had very little possibility to use their hydroplanes to "fly" the submarine towards the surface.
The bathyscape Trieste, made it to 35,813 feet in the Challenger Deep in the Marianas Trench, a few hundred miles east of the Philippines. The ocean is 36,200 ft. deep there. The water pressure at that depth is over 1000 atmospheres. The Trieste was 6.5 ft. in diameter with steel walls 5 inches.
The USS Thresher
At 9:18 am on April 10, 1963, sonar operators aboard the US Navy submarine rescue ship Skylark, which was accompanying the nuclear attack submarine Thresher, heard a sound like air rushing into an air tank when the Thresher went down in 8,400 ft. of water on its deep-dive trials southeast of Cape Cod, Massachusetts, killing all 129 men aboard.
Five minutes prior to the implosion, Thresher had radioed that it was having minor problems then there were several fragmentary, garbled messages, followed by silence. Moments later the sounds of the Thresher breaking apart and imploding were heard.
According to US military reviews of the accident, the most likely explanation is that a piping joint in a sea water system in the engine room gave way with the spray shorting out electronics that in turn forced an automatic shutdown of the nuclear reactor.
When the accident occurred, Thresher was near its maximum test depth which was classified, but probably around 1,300 ft. Normally, a submarine would be able to survive depths 20 to 35 percent greater than its maximum test depth, but without the reactor and any power, the Thresher could not stop itself from sinking. Thresher Memorial