The Etruscan Well beneath Perugia's historic centre is one of the city's most remarkable survivals: a 3rd-century BCE cistern, 37 metres deep, cut directly through the limestone bedrock to supply the Etruscan settlement above. It was discovered in 1782 when a nobleman's cellar was being dug out, and has been accessible to visitors since the 19th century.
The descent by modern staircase takes you down into the rock, following the original shaft. At the bottom, you can look up the full depth of the well (now lit for visitors) and see the original stonework lining still intact. The whole complex — access shaft, main cistern, opposing return shaft — is an impressive piece of ancient engineering for a pre-Roman Italian city. It sits just off Piazza Danti, a short walk from the Fontana Maggiore, and is easily combined with a loop through the nearby lanes.