Pantheon

The Pantheon standing today was built under Emperor Hadrian around 118–128 AD, replacing an earlier temple on the site commissioned by Marcus Agrippa in 27 BC (whose name Hadrian modestly kept inscribed on the facade instead of his own). What makes it extraordinary isn't just its survival — it's the only major building from ancient Rome that has remained continuously in use, first as a pagan temple to all the gods, then, after being consecrated as a church in 609 AD, as a functioning Catholic church, which is almost certainly why it escaped the systematic stone-robbing and quarrying that stripped bare so many other Roman monuments.

The dome itself remains the largest unreinforced concrete dome ever built, a record unbroken for over 1,900 years, achieved through a series of engineering tricks that still impress structural engineers: the concrete mix gets progressively lighter as it rises, using heavier travertine aggregate at the base and volcanic pumice near the top, while coffered recessions reduce weight without sacrificing strength. At the dome's centre, the oculus — a 9-metre circular opening, permanently unglazed — is the building's only light source, and on rainy days water genuinely falls straight through onto the sloped, drained marble floor below, exactly as the architects intended.