Huge Structures On The Moon: Alien Bases Or Secret Space Program?

[eburacum]

One theory I've heard decently is that the maria were formed by a process similar to plate tectonics on Earth, which would lead to polygonal shapes in some cases. The Moon then froze, leaving the ancient crustal features in place, facing Earth.

 

[Xanatic*]

The surface of the moon is largely basalt. A type of rock which often shows hexagonal fracturing.