The largest and youngest of the very large circular impact features on the Moon. It was subsequently flooded by lava to form the dark area known as the Mare Imbrium, which is 1,300 kilometres 800 miles in diameter. The Imbrium Basin is surrounded by three concentric rings of mountains, though only the outer one is defined with any clarity, by the Carpathian, Apennine and Caucasus mountains. The Alps form part of the second ring.