The Karnak Temple area is a vast open-air museum and the largest ancient religious site in the world, and is probably the second most visited historical site in Egypt, second only to the Giza Pyramids near Cairo. The key difference between Karnak and most of the other temples and sites in Egypt is the length of time over which it was developed and used. Construction work began in the 16th century BC. Approximately 30 pharaohs contributed to the buildings, enabling it to reach a size, complexity and diversity not seen elsewhere. Few of the individual features of Karnak are unique, but the size and number of features are overwhelming.