"I am sound that has gone on a journey and returned. Bats use me to hunt. Ships use me to find the bottom of the ocean. I am the voice of the mountain answering back. What am I?"

Answer: An Echo. An echo is sound reflected off a surface back to the listener. Bats use echolocation — emitting ultrasonic pulses and detecting their echoes — to navigate and hunt in total darkness. Sonar uses the same principle to map the ocean floor.