TOEFL【Integrated Writing】TPO32
Starting in the 1960s and continuing until the 1980s, sailors in Russian submarines patrolling the North Atlantic and Arctic Ocean would occasionally hear strange sounds. These underwater noises reminded the submarine crews of frog croaks, so they called the sounds “quackers” (from the Russian word for frog sounds). The sources of the sound seemed to be moving with great speed and agility; however, the submarines’ sonar (a method of detecting objects underwater) was unable to detect any solid moving objects in the area. There are several theories about what might have caused the odd sounds.