Silent Hunter Series/YMMV


 * Goddamned Bats: Aircraft, especially Allied aircraft. Sure they start off as a joke, but by 1944 they can spot you beyond visual range with radar, appear out of nowhere and stick a bomb right on your conning tower on the first pass. The only thing you can do is hit 'crash dive' and pray. And they're everywhere.
 * Hell Is That Noise:
 * Sonar pings and propeller noises when you are getting chased by escorts, especially after a successful attack (as you might lose all that hard work).
 * "Depth charges in the water, sir!"