Vintage photograph. Echo River, 360 ft underground in Mammoth Cave. On the fifth and lowest level of Mammoth Cave, Echo Rivers follows its winding course. Although several miles of the stream have been explored, its origin is still a mystery. The river averages 30 ft in width and the arched ceiling varies from 3 to 20 feet high. The eyeless fish live in the emerald green waters, which is 30 ft deep. A highlight of the boat ride is when guides dim their lanterns and call out harmonious notes in the darkness, as the sounds are amplified and reverberated through the channels producing echoes of weird beauty, hence the name Echo River