Pantheon

Nearly 1,900 years old and still the world's largest unreinforced concrete dome, the Pantheon is Roman engineering at its most audacious. The oculus — a nine-metre circular opening at the dome's apex — is the building's only light source, casting a beam that moves across the interior like a sundial. When it rains, water falls through and drains through almost-invisible holes in the slightly convex floor. Raphael's tomb sits inside, as does the first king of unified Italy. The interior is best appreciated by standing dead centre and looking straight up: the coffered dome creates an optical illusion of even greater height than its 43-metre reality.