Pantheon

The Pantheon is the best-preserved building of ancient Rome because it never stopped being used: consecrated as a church in 609 AD, it skipped the quarrying that dismantled the Forum. What stands today is Hadrian's rebuild of 118–128 AD, though he kept the original founder's inscription — M·AGRIPPA — across the porch, which has confused visitors for nineteen centuries.

The dome is the reason to come. 43.3 metres across and exactly 43.3 metres high — a perfect sphere would fit inside — it remains the largest unreinforced concrete dome ever built, nearly two millennia after its pour. The Romans graded the aggregate from heavy basalt at the base to featherweight pumice at the crown, and the coffers hollow it further. The only light source is the nine-metre oculus, open to the sky: rain falls through onto the (drained) marble floor, and on a sunny day the light disc sweeps the interior like a slow searchlight. Around midday in April and August it aligns dramatically with the entrance door.

Since 2023 there's a €5 entry fee — buy on the official Musei Italiani site or at the (fast-moving) door queue. Inside are Raphael's tomb, with the inscription that "nature feared to be outdone by him while he lived," and the tombs of Italy's first two kings. It remains a consecrated church: Saturday-evening and Sunday-morning mass attendance is free, and the building empties of tour groups by 8:30 AM if you come at opening.

The piazza outside, Piazza della Rotonda, is tourist-priced — walk two minutes to Sant'Eustachio Il Caffè or Caffè Tazza d'Oro for the coffee the neighbourhood is actually famous for.