During a war that probably seemed important at the time, the Ottomans were defending the city against Venice, and they decided to use the Parthenon to store all their gunpowder, possibly in the belief that the Venetians wouldn't want to destroy such an ancient and beautiful structure. It's the same reason the Mona Lisa spent World War I strapped to the front of a battleship. The gunpowder was soon hit by a cannonball, triggering a massive explosion which completely destroyed the inner temple and punched massive holes in the outer structure. The building was left in ruins, and remained that way for the next couple of centuries.
The Parthenon we know now is actually the product of modern reconstruction efforts, which began in earnest in 1975 and continue to this day. And by "reconstruction," we don't just mean sanding off the cruise ship graffiti and taking down the charred "Ottoman Gunpowder Depot: 17 Day Without An Accident" sign. Brand-new marble blocks have been hand-carved with ancient techniques and used to rebuild the damaged walls and columns. It's been a massive undertaking, with craftsmen spending up to three months carving a single block to exactly match its original counterparts. In one case, an entire column was hoisted into the air so they could repair a damaged bottom segment, then lowered back down in the most high-stakes game of Jenga imaginable.
And they're currently planning another project to partially rebuild an obliterated wall of the main inner chamber. At some point, it would have probably been easier to simply leave the ruins intact and build a Parthenon II ("This time we have water slides!") next door.
Alix Lee/Public Domain Pictures