(copied from the server) The two are fundamentally linked, and there's substantial overlap between them already. Almost all analysis about the pit gate is really about the undercave, and there's just not enough stuff that's strictly about the pit gate to justify a separate page - after all, all it really does is sometimes spawn fleshbeasts and act as an entrance to the undercave, and dealing with the pit gate requires going to the undercave anyway.
Since the thing players actually see named is the pit gate (and it's the first thing to pop up) it should probably be the page merged to.