In a suburb of Cincinnati, there is an urban legend called Satans Hollow.

I've posted links below to videos by a couple of folks who have looked into the location.

My Brother in law and I intend to explore these tunnels in the near future. I'll let you know what we find.

Description from Haunted Ohio

Satan's Hollow in Blue Ash (a Cincinnati suburb) is the name of an underground 'altar room,' located in the sewer system, where a group of Satan worshippers supposedly raised demons and even dealt with the Dark One himself. One of his minions, a demon in the form of a 'shadow man,' haunts the underground tunnels where they performed their ceremonies. The screams of a woman are often heard echoing through the passages.

CRAPS Video of satan's hollow

Some kids at night in Satan's Hollow. *Warning Loud Scream Towards The End*

Same Kidz, have a baseball thrown at them from a covered manhole

I make no claims as to the validity of the legend or the videos.

I wrote the guy who shot the second set of videos. He seemed to think that there were just people in the tunnel messing with his group and that he was being herded or was being surrounded the second time he went in. To what end is anybodies guess. I know the location now and as soon as the wet weather breaks and we have a little drying I'm going in with a few friends. I'll post pics and vids as soon as I get them.