You'll need to book (by phone) at least two hours ahead to visit the world's largest Roman cistern, but it's well worth the effort to stand in this underrated ancient wonder. Bathed in an eerie light and featuring 48 soaring pillars and a barrel-vaulted ceiling, the Marvellous Pool is more 'subterranean cathedral' than 'giant water tank'. While there is no entrance fee, a small tip (around €3 per person) is appropriate.
The cistern was an Augustan-era creation, its 12,600-cu-metre water supply serving the military fleet at nearby Miseno. Fresh water flowed into the cistern from the Serino river aqueduct, which was then raised up to the terrace with hydraulic engines, exiting through doors in the central nave. Engineers still marvel at its technical sophistication.