Stadium of Domitian (Piazza Navona substructure)

The original 1st-century AD athletic stadium beneath Piazza Navona — Domitian built it for Greek-style athletic competitions, and it could hold around 30,000 spectators. The piazza's distinctive elongated oval shape preserves the stadium's racetrack footprint, and you can descend into the underground excavations through a small entrance at Via di Tor Sanguigna 13 to walk the actual ancient track. The included audio guide explains how the stadium worked and how the modern square evolved on top.