Piscina Mirabilis

Under a hill above the town of Bacoli, on the western tip of the Bay of Naples, lies a vast underground hall built not for people but for water. Forty-eight cruciform pillars, in four rows of twelve, hold up barrel vaults some fifteen metres high, dividing the space into five long aisles like the nave of a church; light falling through openings in the roof cuts slanting beams through the dim interior. Locals call it the Cathedral of Water. The Piscina Mirabilis, the "wondrous pool", a name traditionally attributed to Petrarch after one of his visits, is one of the largest cisterns the Romans ever built.

It dates from the reign of Augustus and was filled by the Aqua Augusta, the aqueduct built after 33 BC to bring spring water to the towns and villas around the Bay of Naples. The usual explanation is that it supplied the Classis Misenensis, the Roman fleet based in the nearby harbour of Misenum, which needed large amounts of fresh water for its men and ships. Because it stands about a kilometre from the naval quarters, which were fed directly from the aqueduct, some scholars think it may instead have served one of the luxurious villas that lined this coast, as the smaller cisterns nearby did.

The hall is about 72 metres long, 25 metres wide and 15 metres high and held some 12,600 cubic metres of water, roughly the volume of five Olympic swimming pools. The walls and pillars are faced with net-patterned opus reticulatum, and their surfaces were sealed with waterproof cocciopesto, with the corners rounded to prevent leaks. A shallow basin sunk into the floor of the central aisle collected sediment so the cistern could be cleaned and drained, and water was drawn up through openings in the vaults by machines working from the roof terrace. In the late 1st or early 2nd century a row of twelve small vaulted chambers was added along one side to increase capacity. The cistern fell out of use when the Aqua Augusta failed in the 4th or 5th century. It was cleared and consolidated in the early 20th century, its vaults were partly rebuilt in 1936, and the roof was waterproofed again in 2007.

Getting there

The Piscina Mirabilis is in the town of Bacoli, on the Miseno peninsula at the western end of the Gulf of Naples, near the ancient harbour of Misenum. Naples International Airport (NAP) is about 20 km away.