For centuries, speculation and scientific investigation have been centred on the Moon; it was the first new world to be visited by humans. Although many questions remain about lunar history, it is clear that the Moon holds keys to understanding the origin of the solar system. The Moon is a spherical, rocky body, possibly with a small metal core, orbiting the Earth in a slightly eccentric orbit at a mean distance of near 400,000 kilometres. Its radius is about 1,738 kilometres, and its shape is slightly flattened, with its longest axis along the radial direction from the Earth. Its mass distribution is not uniform; the centre of mass is displaced about two kilometres toward the Earth relative to the centre of the lunar sphere, and there also are surface mass concentrations, called mascons, that cause the Moon's gravitational field to increase over local areas. The Moon has no global magnetic field like that of the Earth, but some of its surface rocks have remanent magnetism, indicating one or more magnetic episodes in the past. The Moon presently has very slight seismic activity and little heat flow, indications that most internal activity ceased long ago. It is now known that billions of years ago the Moon was subject to violent heating--resulting in a differentiated crust--followed later by volcanic outpourings of lava. The Moon's mean density is 3.34 grams per cubic centimetre, close to that of the Earth's mantle. Because of the Moon's small size and mass, its surface gravity is only about one-sixth of the planet's and it retains little atmosphere. The molecules of any gases present on the surface move without collision, and so there is no shield to protect the surface from meteoritic and particulate bombardment. As a consequence, countless bodies have struck and cratered the Moon, forming a regolith consisting of rock fragments of all sizes down to the finest dust. In the ancient past giant impacts made great basins, some of which were later partly filled by enormous lava floods. These great dark plains, called maria, are clearly visible to the naked eye from the Earth. The dark maria and the lighter highlands constitute the two main kinds of lunar territory. The mascons are regions where dense lavas rose up from the mantle and flooded into basins. Lunar mountains, mostly along the rims of ancient basins, are tall but not steep or sharp-peaked, because all lunar landforms have been eroded by the unending rain of impacts.