An echo can be heard in a cave when a sound reflects off surrounding walls and then is heard again. It is also possible to hear an echo in an empty room, in a mountain valley, under a bridge or by calling down into a well.
The farther away the reflecting source, the longer it takes the sound to echo back. The word "echo" originates from the Greek "echo," meaning "repeated sound." In Greek mythology, Echo was a nymph who left behind only her voice when she died after her love for Narcissus, a beautiful boy who loved no one but himself, went unrequited.