message in a bottle

Short definition from Wiktionary

Noun

1. 1
   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.
2. 2
   An analogous message sent off into the world or universe without knowledge of who the recipient will be: plaques on craft launched into outer space, interstellar radio messages, stationary time capsules, balloon mail, and so on.