Lurking at the center of many galaxies are supermassive black holes. If there is material nearby, the black hole can gravitationally trap and accrete this matter, releasing a huge amount of energy. The accretion process is inherently unstable, and so the luminosity fluctuates erratically. Surrounding the black hole is a volume of gas called the ``Broad-Line Region'' (BLR). The BLR is photoionized by the high energy radiation produced very close the to black hole. Since this photoionizing flux is variable, the BLR emission also varies in response -- but with a delay. This delay is caused by the time it takes light to travel across the BLR. In essence, the BLR produces emission-line ``light echoes''. Just as with radar, sonar, or echolocation used by bats, the echoes can be used to determine the relative positions and velocities of gas in the BLR. The ``echo mapping'' technique is immensely powerful, but the results so far have been modest due to relatively low quality data. The ``HET Echo Mapping Project'' is a new research program that will use the 9-m ``HET'' telescope to obtain spectacular data. This will allow us to map the size, geometry and flow of the BLR gas, and hence measure the properties of the supermassive black hole and its environment.