Cathedral in the mine
This is the chapel of the cathedral in the Wieliczka Saltmine, Krakow. The mine is so deep that the workers built homes in the mines (the mine is just over 300m deep; by modern standards it is not deep at all, but at the time that was a long way down), and being religious people they needed places to pray. So they build a cathedral under the ground. This is the front of the cathedral, the “choir” from where the services were conducted.