9am-12.30pm, 3pm-6pm, Sun 12.15pm-6pm (last admission to lower basilica 30min before closing)
Founded in the 4C and dedicated to St Clement, this is one of Rome's earliest basilicas. Badly damaged in 1084 it was rebuilt, again on a basilical plan with three naves in the 12C. The upper basilica's apse has extraordinarily colourful mosaicwork. Head down into the lower basilica which retains frescoes, some of which almost resemble ancestors of today's cartons. Even lower down is a 1C house adapted as a mithraeum (temple to Mithras).