The order given by the captain of a submarine
when he wants to rest his boat on the ocean floor. This is a very delicate maneuver due to the potential damage than can be done to the submarine. This is an old tactic dating back to World War II
to avoid detection by active sonar
(you know that thing that goes "ping" that has only been used properly in about three movies).
It works like this, active sonar (Ping!) "paints" a picture of everything around a submarine by sending out a pulse of sound and listening to the echo. Water is a very good medium for sound, so don't close your eyes and try to "see" by shouting "Ping!" alot, you will only get punched for your trouble. Anyway, when a submarine is "bottomed" the sonar of the searching vessel sees it as part of the ocean floor. Just hope no one goes over the top of you with a MAD detector.