The Temple of Olympian Zeus is a colossal ruined temple in the center of Athens that was formerly dedicated to Zeus, king of the Olympian gods. Construction began in the 6th century BC during the rule of the Athenian tyrants, who envisaged building the greatest temple in the ancient world, but it was not completed until the reign of the Roman Emperor Hadrian in the 2nd century AD some 650 years after the project had begun. During the Roman periods it was renowned as the largest temple in Greece and housed one of the largest cult statues in the ancient world.
The temple’s glory was shortlived, as it fell into disuse after being pillaged in a barbarian invasion in the 3rd century AD. It was probably never repaired and was reduced to ruins thereafter. In the centuries after the fall of the Roman Empire, the temple was extensively quarried for building materials to supply building projects elsewhere in the city. Despite this, substantial remains remain visible today and it continues to be a major tourist attraction (I love Wikipedia).