Although humans have been to the Moon, exploration consisted only of six short expeditions, all near the equator on the near side of the Moon. These missions helped answer questions such as how the Moon originated, the composition of its crust (at least in the locations sampled), and its interior (by detecting moonquakes and the response to impacts of spent rocket stages by seismographs deployed on the surface). But many mysteries remain, some dating from observations from the Earth from decades before the first Moon landing.