“The deep sound channel was exploited to set up the SOFAR (sound fixing and ranging) system, which was initiated in 1960 by the Australia-Bermuda Sound Transmission Experiment, in which explosions were set off near Heard Island in the Indian Ocean off the coast of Australia. They were detected in Bermuda, at a distance of 20,000 km. A new, unexpected sound was discovered by the SOFAR researchers, and later identified as the calls of fin whales (Balaenoptera physalus), who long ago discovered the existence and properties of the deep sound channel and regularly visit it to signal their distant kin.”