A microphone lowered into the water. (Into the Jaws of Death)
The microphone is a device used to record audio. Nigel Marven used an underwater microphone to record the calls of a Basilosaurus.
In Sea Monsters
102. Into the Jaws of Death
The crew of The Ancient Mariner lowered an underwater microphone into the Eocene seas to record the calls of the Basilosaurus. Although their first attempt failed to capture any sounds, the following attempt revealed the calls of a Basilosaurus.