The partially excavated ruins of Nero's legendary Golden House, an enormous pleasure palace built after the fire of 64 AD. After Nero's death, the building was systematically buried and built over — the Colosseum stands on what was Nero's artificial lake. What remains today are the subterranean rooms of one wing, covering about five hectares, with fragmentary frescoes visible throughout.
Visits are guided tours only, typically running on weekends. The excavations are ongoing and the site feels genuinely archaeological — half-unearthed columns, roof collapses, original floor mosaics. One of Rome's more unusual experiences precisely because it is still being uncovered. Book ahead.