Black holes may be heard but not seen. As the death state of a star, black holes emit no light. They are dark against the dark sky. But like mallets on a drum, black holes can ring out a song on space itself in the form of gravitational waves. Monumental Earth-based detectors such as LIGO and the planned space-based mission LISA aim to record the songs from space for the first time thereby turning up the volume on the soundtrack to the Universe. There is no sound in empty space. But when the gravitational waves hit the Earth, detectors in the next few years will measure the stretching and shrinking of space and will be able to amplify the result as sound. To hear the sounds of two black holes ringing space, play the movie at the left.