The Stadium of Domitian
The Stadium of Domitian was commissioned by the Emperor Titus Flavius Domitianus in 80 AD (the same year he also completed the Colosseum). It's located to the north of Rome's Campus Martius and it was Rome’s first permanent venue for athletics. It had space for around 20,000 spectators.
This impressive UNESCO Site is linked to Piazza Navona, lying 4.5 meters underneath. It has only been open to the public since 2014.