Pantheon (Basilica di Santa Maria ad Martyres)

Originally a 2nd-century Roman temple, consecrated as a Christian church in 609 AD as Santa Maria ad Martyres — which is the only reason it survived intact while other ancient temples were stripped or demolished. The 43.3-metre concrete dome remains the largest unreinforced concrete dome in the world after almost 1,900 years, and the open oculus at its centre lets natural light, rain, and even the occasional snowflake fall onto the marble floor (where ancient drainage carries water away). Holds the tombs of the painter Raphael (with a Latin epitaph by Bembo claiming Nature herself feared being outdone by him) and Italy's first kings, Vittorio Emanuele II and Umberto I.