Piscina Mirabilis

The Piscina Mirabilis is a large Roman subterranean cistern at Bacoli in Campania, Italy, built in the early Imperial period (1st century AD) to store fresh water. It is one of the largest surviving Roman cisterns on the Italian peninsula.

The interior is a spacious vaulted brick-and-concrete hall supported by rows of square pillars, with a raised central walkway and narrow side passages. The chamber measures about 70 m long and 25 m wide and reaches up to roughly 15 m in height, creating a dramatic, echoing space below ground.

Constructed in the 1st century AD to supply water to the Roman naval base at Misenum (Classis Misenensis), the Piscina Mirabilis served as a pressurized freshwater reservoir for the fleet and nearby settlements. Its solid Roman engineering and waterproof opus caementicium walls preserved the cistern in good condition compared with many other Roman hydraulic works.

The site sits near the town of Bacoli on the Phlegrean Peninsula, close to the archaeological area of Misenum and the Bay of Naples, roughly 20 km west-northwest of the city of Naples.

• Visitor note: The cistern's interior is a single vaulted hall with a raised central walkway and low lighting; sturdy shoes and a light jacket are recommended for the cool, damp interior.