From Church Pop:
More than 300 feet underground in the Wieliczka Mines in Poland lies the Chapel of St. Kinga, the largest underground church in the world.
This amazing construction made entirely of salt welcomes visitors from all over the world. Officially opened in the 13th century, the Wieliczka Mines in Krakow reach a depth of over 1,000 feet. The underground network comprises over 150 miles of galleries, spread over nine levels. This labyrinth of salt is so large that only two percent of its dimensions can be explored.