Named for Emperor Caracalla, a particularly unpleasant individual, the baths were completed in A.D. 217 after his death. The richness of decoration has faded, but the massive brick ruins and the mosaic fragments that remain give modern visitors an idea of the complex’s scale and grandeur. In their heyday, the baths sprawled across 11 ha (27 acres) and included hot, cold, and tepid pools, as well as a palestra (gym) and changing rooms. A museum in the tunnels below the complex—built over an even more ancient mithraem, a worship site of an eastern cult—explores the hydraulic and heating systems (and slave power) needed to serve 8,000 or so Romans per day. Summer operatic performances here are an ethereal treat.