Some 1,400 bomb shelters were built across Barcelona during the Spanish Civil War (1936–1939) to protect its residents from Fascist air strikes. Among the handful that survive, Refugi 307 is the largest and best preserved. Its three entrances and 650 feet of tunnels were carved into the sandstone hill of Montjuïc; the shelter could hold up to 2,000 people during an air raid and was one of the few with running water, toilets, and an infirmary. The only way to visit Refugi 307 is with a guide, who will put the history of the civil war into context before leading you through the network of passageways. Tours are conducted on Sundays, in English, at 10:30 a.m., and you’ll need to make a reservation by emailing email@example.com and specifying “English tour” in the subject line. The cost is 4 euros (around $4.50).