Located in an actual cave, this frenetic disco heats up (literally – there's little air circulation down there) around midnight. Entry includes one drink; it's a good mix of locals and tourists. To get there, follow Calle Bolívar from Plaza Mayor up to the Ermita de Nuestra Señora de la Candelaria de la Popa. It's 100m further along on your left.
Hang onto your ticket for reentry privileges, as you might want to resurface for fresh air at some point.