Piazza Navona.

Bernini unveiled it on 12 June 1651, and the trick still works. A 16-metre red granite obelisk, hauled in from the Circus of Maxentius on the Appian Way, balances on a hollowed travertine grotto so the whole multi-ton spike appears to float. Walk around the base — most visitors photograph the front and miss the gap underneath.

Four river gods sprawl on the rock: Danube with the Pamphilj arms, Ganges with his oar, veiled Nile, and a bald Río de la Plata clutching coins beside an armadillo Bernini invented from hearsay. The horse next to the Danube and the lion drinking from the Nile are the master's own chisel work; the rest came from his workshop. Guides will tell you the Plata's raised hand shields him from Borromini's Sant'Ignazio-style facade across the way — charming, but the fountain was finished before Sant'Agnese was even designed.

Look up. Tiny dove with an olive branch on the obelisk's apex. Holy Spirit and Pamphilj crest in one bird, and almost nobody sees it.

Borromini's concave facade pulls you in like a slow inhale. The twin bell towers frame the dome so it reads as part of the front itself, and the window beneath the left campanile isn't church at all — it belongs to Palazzo Pamphilj next door, an architectural cheat that Sorrentino's cameras lingered on in La Grande Bellezza.

Inside, the Greek-cross plan opens under an octagon banded in peach-pink persichino and red Cottanello marble. The verde antico columns at the high altar were salvaged from the Arch of Marcus Aurelius. Baciccio painted the four cardinal virtues into the pendentives between 1666 and 1672, and the dome above shows Agnes carried into paradise by Ciro Ferri. Clap once: the reverb runs five seconds.

Duck down to the crypt. A medieval oratory sits beneath the marble, lit recently by a Webuild donation, and Algardi's relief of Agnes's miraculous hair — she was martyred at thirteen on this exact spot in 304 AD, when the stadium's outer arches housed a brothel — is almost always alone with you.

The piazza's oval isn't a coincidence — you're standing six metres above Domitian's stadium of 80 AD, the only masonry athletics arena Rome ever built, and the entrance at the north end drops you straight into it.

Travertine arches of the curved end still stand in opus latericium brick. The temperature falls to a steady 15°C, the street noise vanishes, and on a few unprotected thresholds you can see grooves polished smooth by athletes' feet entering the carceres. Open daily 10–19, last entry 18:20, and it's quiet even when the piazza above is packed shoulder to shoulder.