The Olympeion was the largest ancient temple on the Greek mainland—a rectangular structure almost 100 metres long and nearly 50 metres wide. It was laid around 515 BC by Peisistratis the Younger on the foundations of a smaller temple. Most of its original 104 columns were dismantled or destroyed as the site was used as a quarry in medieval times. By the 15th century, only 20 columns were still standing. Today, 15 standing columns and one fallen one remain.