message in a bottle

A message written on a scrap of paper, rolled-up and put in an empty bottle and set adrift on the ocean; traditionally, a method used by castaways to advertise their distress to the outside world.

(figuratively) An analogous message sent off into the world or universe without knowledge of who the recipient will be (although not necessarily without a particular intended recipient); thus, plaques on craft launched into outer space, interstellar radio messages, stationary time capsules, balloon mail, and so on.