A simple device for signalling the arrival of a deep cast on bottom has been developed and is now in routine use. The device is used either in line with corers or is suspended below as a pilot. When contact of either the ball-breaker or other apparatus is made with the bottom, a small glass sphere in the former is imploded and the resultant signal at the surface is amplified and reproduced over a loud speaker system. The device has been used successfully at depths to 2,700 fathoms.
Issacs, John D., and A. E. Maxwell. 1952. "The ball-breaker, a deep water bottom signalling device." Journal of Marine Research 11, (1). https://elischolar.library.yale.edu/journal_of_marine_research/759