A cave, probably in new england, uncovered during the mining taking place around there about two-hundred years ago.
When walked through has non-normal geometry in the form of a single extra tunnel leading to the surface which can't be found from outside.
Going into the tunnel means coming out in an apparently very large "few dozen kilometers bordered by [DATA EXPUNGED]" area full of temperate forest conditions. No abnormal creatures in it, just stuff which descended from ordinary forest life having wandered into the gate. The twist idea is simply that the foundation proceeds to set up a site inside the area with intent of having it be able to ride out a class-XK event.
So, what do you think of this idea?