Catacombs at Tuna el-Gebel
The site of Tuna el-Gebel in Middle Egypt was known to the Greeks as Hermopolis. In the southern part of the site there is an animal necropolis. Mummified ibises and baboons, associated with the god Thoth, were buried here in their thousands in subterranean galleries, similar to the burials of the Apis bulls at Saqqara.
The Saqqara plateau served as a burial site to the ancient Egyptians for over three thousand years. It is home to pyramids, private tombs and temples, and is even the burial place of sacred animals. The most famous of the animals buried at Saqqara were the Apis bulls. For over a thousand years these bulls were laid to rest in the darkness of the Serapeum, a massive gallery of tunnels and niches carved into the rock below Saqqara.
Source and more info: Saving the Serapeum (via Dr Hawass).