These are the famous 'mare' which form the 'Man in the Moon'. They represent dark, basaltic material like earth lava, that flowed out of the lunar interior when the Moon was only about 3.3 billion years ago, although Apollo astronauts brought back rock samples substantially younger than this. It is thought that soon after the Moon was accreted and its crust solidified, its interior was melted by radioactive heating and lava escaped to the surface through deep cracks. These cracks were produced during the Late Bombardment Era when very large asteroids perhaps hundreds of miles across impacted the Moon to form the basins we now see such as Mare Imbrium. The dark lava flooded these impact basins to form the 'seas'.