Earthquakes and the seismic waves they generate provide some of the best clues to Earth's interior. To extend that success to other worlds, scientists placed seismometers on the Apollo missions to the moon and on the Viking Mars landers, which reached Mars in 1976. Though the Apollo detectors proved a resounding success, Viking's results on Mars were less clear: Viking 1's seismometer failed to deploy properly, and Viking 2 recorded only two ambiguous events.