Ocean at Moon
Sorry, but water filled seas do not exist on moon. First astronomers saw the moon as a counterpart to earth and compared the large dark areas to our oceans. However, mare actually are relatively plain basalt surfaces and have an average age of about 3.1 billion years (estimated moon age: 4,5 billion years). These areas cover about 17 % of the lunar surface and they are found more frequently at the side turned to earth. It is assumed, that enormous meteorites collided with the moon. From the impact holes lava rose, flooded the crater and solidified afterwards.