I’ve heard the following from more than one person.
There’s that winding road up to Magashegy, a mountain in my hometown. Not long after you pass the first big curve down below, there are some plots of land. About 150 meters further on, somewhere on the left, there is said to be a cave. A cave where, many years ago, workers went down and experienced some very strange things.
Their flashlights failed, and they heard such terrifying sounds that they could not describe them. But they weren’t the only group to go down there; there were others—and if I recall the rumors correctly, some of them even disappeared.
And of course, there are conspiracy theories about the phenomenon—from dimensional portals to the hollow Earth, to secret government experiments—almost everything you can imagine. That is, if these anecdotes are true at all.
I do not know if these incidents are true but it may be worth investigating.