L'acqua sgorgava dalle viscere della terra, calda e fredda riempiendo antiche sale termali, cisterne e conche di millenari crateri, coprendo nel tempo parte del territorio dei Campi Flegrei e custodendo in profondità testimonianze archeologiche oggi protette dal Parco Archeologico di Baia Sommersa.
The Piscina Mirabilis was constructed to give place to the imperial fleet staying in the harbour of Miseno, the Classis Misenensis. It's the largest roman cistern in Italy completely carved out of the tufo, terminal point of the Augustan aqueduct which now presents itself as an enormous subterranean cathedral.
The ancient Baia, for the precence of thermal waters and the beauty of it's landscape, was a resort for rest and vacation of the roman aristocracy. Between the third and the fifth centuries A.D.the phenomena of Bradyeism provoked this slow sinking of it's docks, villas and edifices and it contributed to the depopulation of the area. A part of the ancient city is now under water and is part of the Underwater Park of Baia protecting a valuable marine archeologichal ambience.