Read the passage and answer. When, during the first quarter of the twentieth century, echo sounding was developed to allow ships while underway to record the depth of the bottom, probably no one suspected that it would also provide a means of learning something about deep-sea life. But operators of the new instruments soon discovered that the sound waves, directed downward from the ship like a beam of light, were reflected back from any solid object they met. Answering echoes were returned from intermediate depths, presumably from schools of fish, whales or submarines: then a second echo was received from the bottom. For what purpose was the instrument originally designed?