Hadley Rille is a kilometre and a half wide and three hundred metres deep, and it meanders like a river — but nothing has ever flowed through it except lava.

Sinuous rilles are collapsed lava tubes or channels cut by molten rock, and Apollo 15 landed beside this one specifically to look into it. The crew photographed layered bedrock in the far wall, which is the closest anyone has come to reading the Moon's geology in cross-section.