largest lunar crater

The largest structure on the Moon listed officially as a crater is Hertzsprung, diameter 591 km, which lies on the farside of the Moon and so is permanently invisible from Earth. It is a multi- ringed impact feature. Similar impact structures on the nearside of the Moon became flooded with lava which solidified into dark rock. These features are now named as maria (seas) rather than craters. Such volcanic flooding did not occur on the lunar farside. As a result, there are many more very large impact structures on the farside that are described as 'craters' compared with the nearside. They include Apollo (537 km), Birkhoff (345 km), Korolev (437 km), Mendeleev (313 km), Planck (314 km) and Schroedinger (312 km). The largest crater on the nearside of the Moon is Bailly, diameter 287 km (not to be confused with Baily, which is a different crater). The largest impact basin of any kind identified on the Moon is the South Pole-Aitken Basin, which is 2,500 km across and stretches almost a quarter the way round the Moon.