Normally I don't put real world stuff on the internet, but on my vacation I stopped for a tour of an active salt mine. The part we toured is essentially done with mining, but is connected to the active mine. The whole place was built using the room and pillar method (leaving a giant cavern with evenly spaced 40x40 ft. 'pillars').
The whole thing was amazing and inspiring. I didn't think much about D&D while underground, but at one point, I thought it would be an awesome game map. Getting to see actual caverns definitely will change my take on the 'dungeon' part of the game.Salt Mine
someone's blog about it: 1
and 2room and pillar mining method