You've probably seen the mysterious giant hole that was discovered in Siberia, which appears to lead into a large cave. We don't know its origin yet, but it isn't the only hole like that on Earth. Or the moon: NASA found these formations there too, which are perfect for future human outposts. Or current alien/Nazi UFO bases.
Here are some high resolution images of the lunar pits. NASA estimates that there are over two hundred pits on the moon's surface. These shouldn't be confused with the millions of craters on the moon's surface though because they're something different. The pits range from sizes of about 5m to 900m and get even wider underground. NASA explains how they form:
The pits could form when the roof of a void or cave collapses, perhaps from the vibrations generated by a nearby meteorite impact, according to Wagner. However, he noted that from their appearance in the LRO photos alone, there is little evidence to point to any particular cause. The voids could be created when molten rock flowed under the lunar surface; on Earth, lava tubes form when magma flows beneath a solidified crust and later drains away. The same process could happen on the moon, especially in a large impact crater, the interior of which can take hundreds of thousands of years to cool, according to Wagner. After an impact crater forms, the sides slump under lunar gravity, pushing up the crater's floor and perhaps causing magma to flow under the surface, forming voids in places where it drains away.
It's pretty impressive how much of a perfect circle some of them are.