Saqqara holds both the oldest large stone building in the world and an underground gallery built for the burials of bulls treated as living gods.
Older than the pyramids at Giza
The Step Pyramid of Djoser, designed for the Third Dynasty pharaoh by his official Imhotep, is generally regarded as the oldest colossal stone building in the world, predating the Giza pyramids by several generations and marking the transition from earlier mudbrick royal tombs to monumental stone construction, a shift credited largely to Imhotep's own innovation.
Burying a bull as a god
The Serapeum, a network of underground galleries at Saqqara, held the mummified remains of the Apis bulls, animals worshipped in life as living manifestations of the god Ptah and, after death, associated with Osiris, each interred in an enormous polished granite sarcophagus weighing many tonnes, a scale of animal burial without a close parallel elsewhere in the ancient world.
Found fast, at a cost
As this entry's companion article notes, the Serapeum's rediscovery by Auguste Mariette in the 1850s, guided partly by the ancient geographer Strabo's description, was carried out with a speed that came at the cost of careful documentation, and some archaeological context was lost or damaged in the rush to excavate before French sponsors' patience, and funding, ran out.