This natural feature of the Mire seems to have been used by cultists for their strange, unspeakable rituals.
Big Maw is a giant hole in the ground with a small pond at the bottom. There are stairs leading to the surface of the pond. At the bottom of the pond, there are two cultist statues overlooking a sacrificial altar and what appears to be the skeleton of some prehistoric creature.
Another spring - Note, on a picnic table.
Big Maw appears only in Fallout 76.