This freshwater cistern is a marvel of Roman engineering. The cavernous place was carved entirely out of the tuff rock cliff. It was made famous in the book Pompeii by Robert Harris. The water from this Augustan-era cistern supplied the Roman fleet camped at Misenum (Miseno).
Guidebooks will tell you to call ahead and that certainly works. However, if you find yourself in the area and want a visit - simply walk up the staircase labeled "Piscina Mirabilis" and knock on house number 6. The occupants are the caretakers and they can help you gain access. The are always inviting and hospitable. If possible spend time in this marvelous town - great shopping, cafes, and outdoor spaces!