what causes a echo ?

Answer 1

A true echo is a single reflection of the sound source. The true cause of an echo is naturally the sound that starts (in the earth's atmosphere: a voice, tree falling, whatever or underwater: a dolphin's whistle, rockslide). Then that sound is reflected off a surface and returns to the listener.

Answer 2

What causes an echo?

http://www.worsleyschool.net/science/files/echo/echo.html
http://www.alectrosystems.com/Acoustics/RoomEcho.htm
http://en.wikipedia.org/wiki/Echo_%28phenomenon%29