Diocletian's largest single project in Rome was the great baths that bear his name on the Viminal Hill (ca. AD 298-ca. 306). Their layout owes much to the Baths of Caracalla, although they are even larger in scale. The central bathing block is a building of considerable complexity with its changing rooms, open-air swimming pool, bathing halls, hot and cold pools, palaestrae (exercise grounds) and warren of service corridors and furnace rooms. Parts of the frigidarium were transformed by Michelangelo into the church of S Maria degli Angeli (1566), with the result that some of the original spatial and lighting effects of the interior can still be appreciated.