Baths of Diocletian

The largest bath complex of antiquity (298–306 AD), built by 40,000 forced labourers and once covering 13 hectares — large enough to bathe 3,000 people simultaneously. After the aqueducts were cut by invading Goths in 537 AD, the baths fell into ruin, but their bones survive: Michelangelo converted the central frigidarium into the church of Santa Maria degli Angeli (preserving the original Roman vaults), and added a serene cloister that's still part of the National Roman Museum housed here today.