Barcelona History Museum (MUHBA, Plaça del Rei)

MUHBA's flagship site sits underneath Plaça del Rei, and what makes it remarkable isn't the square above — a genuinely handsome medieval space in its own right — but what lies beneath it: roughly 4,000 square metres of excavated Roman Barcino, the colony founded around 15 BC that Barcelona grew from, uncovered during 1930s construction work and gradually opened to visitors via a network of walkways and viewing platforms suspended just above the original ground level.

Walking the underground route, visitors pass the remains of Roman houses, a laundry and dye workshop, wine-making vats, defensive walls and a public bath complex, laid out much as they would have functioned in the 1st to 6th centuries AD — a level of preserved urban infrastructure that's genuinely rare to see this completely and this centrally in any European city, let alone directly underneath a still-functioning historic square.