Sofar bomb
In oceanography, a sofar bomb (Sound Fixing And Ranging bomb), occasionally referred to as a sofar disc, is a long-range position-fixing system that uses impulsive sounds in the deep sound channel (SOFAR channel) of the ocean to enable pinpointing of the location of ships or crashed planes. The deep sound channel is ideal for the device, as the minimum speed of sound at that depth improves the signal's traveling ability.