Because of libration effects, only about 10 per cent of the Moon's farside may be observed from Earth. About 40 per cent of the Moon's surface, however, remains permanently hidden.

The first, rather blurry views of the Moon's farside were obtained by the Soviet Moon probe Luna 3 in October 1959. Although they may have been unsharp and blurred, these pictures showed, however, that the surface of the farside differed greatly from the nearside. On the farside, for example, there are only four small maria, and a meagre half-dozen large craters whose floors are lava-flooded.

The ratio of the areas of mare surface to the crater-saturated highlands on the Moon's nearside amounts to about 31.2 per cent to 68.8 per cent, and thus roughly 1:2.2. On the farside the ratio is, however, 2.6 per cent to 97.4 per cent, so roughly 1:37.5. Almost the whole of the farside of the Moon lies well above the average level of the surface.

One reason for this is undoubtedly the fact that the crust on the farside, at up to 140 km thick, is nearly double the thickness of the crust on the nearside. What led to this striking difference is still under discussion by planetologists and geologists.

But the fact is that impact events would have had to have been significantly more violent on the farside to break through the crust to produce cracks and fissures through which lava could flow. As such, the distance that the mantle lava would have to rise vertically would be twice that on the nearside.