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!"