"Take the High Road"
Per the marker posted nearby this entrance, this is the entrance to a 511 foot shaft that Lieutenant Colonel Henry Pleasants ordered to be dug. Members of Pleasants 48th Pennsylvania, many of whom were Pennsylvania coal miners, spent a month digging the shaft. The subsequent use of the shaft led to a monumental aspect of the Civil War in Petersburg, VA.
Today it appears to be quite tranquil and possibly just another route to the other side of the hill.