What was the Parthenon being used for when it exploded in 1687?

A gunpowder magazine. The Ottomans, besieged in Athens by a Venetian army, stored munitions in the temple on the reasonable assumption that nobody would shell a building that old. A Venetian mortar shell went through the roof, and the middle of the Parthenon - which had stood for 2,100 years - was blown out in an instant.

It had survived by being continuously useful. It was a temple to Athena, then a Christian church, then a mosque, and each conversion altered rather than demolished it. The explosion did more damage in a moment than two millennia of everything else.

The surviving sculptures were then removed. Lord Elgin took roughly half of the frieze to Britain between 1801 and 1812 under a permit from the Ottoman authorities whose scope has been argued over ever since, and sold them to the British Museum after going bankrupt. The case for keeping them has shifted over the years - originally that Greece could not care for them, which the purpose-built Acropolis Museum answered in 2009 - and the argument now is largely about precedent. The building also has almost no straight lines: the columns bulge slightly and lean inward, and the platform curves upward in the middle, all to correct for how the eye would otherwise see it.