When the invention of the aqualung first freed divers from the restrictions of fixed rope and airline, it introduced a communications problem between divers and their surface support teams. We now take radio comms for granted, but in 1944 a French diver, Jacques Lacustrine, came up with an ingenious system involving cork spheres. The spheres were attached to a special belt by metal studs.
The studs were weighted so as to exactly counterbalance the buoyancy of the cork spheres they secured. When a diver freed his balls the securing studs fell away, making the system buoyancy neutral. Watchers above 'read' the balls according to a prearranged code as they popped to the surface, and in turn communicated with the diver by dropping clay spheres which had trailing ribbons attached to make them more visible.
Lacustrine's system proved cumbersome and soon became redundant. In particular, the cork balls easily became accidentally dislodged, sending unintentional messages at inopportune moments. It did, however, give rise to the phrase, "I don't pop my cork for every manatee".