As a kid I heard a story of all the empty rooms and strange tunnels under Stockholm. I've been a bit curious ever since. It's said it looks like a swiss cheese underneath. Lot's of old things, sewers, unfinished subway tunnels, pumping stations, cable tunnels, and old reactor hall (!) and what not. I heard they have recently replaced the oldest gas pipes, laid in the 1850's. IIRC it was a company from London that built the first gas distribution system in Stockholm.
