Final answer:
Hydrophones are the underwater microphones used to record whale sounds for marine research.
Step-by-step explanation:
The underwater microphones that record whale sounds are known as hydrophones. These devices operate on the principle of sonar, where a transducer sends out a sound wave, reflects off an object, and the return time of the sound wave is used to determine the distance to the object since the speed of sound is constant. Hydrophones are essential for scientific research, allowing researchers to monitor marine life and study the complex communication of whales and other sea creatures.