Wow, you can really zoom in on that image.

The Maria are giant lava fields. The lava flowed over the moon dust regolith. They appear lower than the regolith layer (like a dry sea) because the molten lava melted the regolith dust under it. The Maria only exist on this human eye visible side of the Moon. There are no Maria on the far side, only regolith dust and impact craters. All the anthropomorphic projection transfer that humans have imagined as the man, the rabbit, all the faces, are lava fields. Along the perimeters, there are high concentrations of 15% titanium. We actually don't need to mine an asteroid, there was a period of time several billion years ago, that the surface of the Moon was pounded with asteroids, meteors and comets. All of their material is in the regolith.