Here's from the movie The Enemy Below(1957). You can watch it on youtube.
It might be a fishing smack or... well, it might be the conning tower of a sub, sir. - Bridge. Lieutenant Ware. - Mr Ware. We're about to lose radar contact at the rate we're going. Reduce speed to speed of target and get on its tail. At this distance, we'll make visual contact at daybreak. - Shall I shake the ship out for a standby? - Not much shaking out to be done. But ready underwater search gear. If this spook is a U-boat, he might pull the plug, and we'll have to go after him.
What does "pull the plug" mean?