From Wikipedia

Ancient Egyptians
The Twins' Tale

The Serapeum of Saqqara is a serapeum located north west of the Pyramid of Djoser at Saqqara, a necropolis near Memphis. It was the burial place of the Apis bulls, living manifestations of the god Ptah. It was believed that the bulls became immortal after death as Osiris Apis, shortened to Serapis in the Hellenic period. The most ancient burials found at this site date back to the reign of Amenhotep III.

In the 13th century BCE, Khaemweset, son of Ramesses II, ordered that a tunnel be excavated through one of the mountains, with side chambers designed to contain large granite sarcophagi weighing up to 70 tonnes each, which held the mummified remains of the bulls. A second tunnel, approximately 350 m in length, 5 m tall and 3 m wide (1,150×16×9.8 ft), was excavated under Psamtik I and later used by the Ptolemaic dynasty.
The long boulevard leading to the ceremonial site, flanked by 600 sphinxes, was likely built under Nectanebo I.

The temple was discovered by Auguste Mariette, who had gone to Egypt to collect coptic manuscripts but later grew interested in the remains of the Saqqara necropolis. In 1850, Mariette found the head of one sphinx sticking out of the shifting desert sand dunes, cleared the sand, and followed the boulevard to the site. After using explosives to clear rocks blocking the entrance to the catacomb, he excavated most of the complex. Unfortunately, his notes of the excavation were lost, which has complicated the use of these burials in establishing Egyptian chronology.
Mariette found one undisturbed burial, which is now at the Agricultural Museum in Cairo. The other 24 sarcophagi, of the bulls, had been robbed.

A controversial aspect of the Saqqara find is that for the period between the reign of Ramesses XI and the 23rd year of the reign of Osorkon II – about 250 years, only nine burials have been discovered, including three sarcophagi Mariette reported to have identified in a chamber too dangerous to excavate, which have not been located since. Because the average lifespan of a bull was between 25 and 28 years, egyptologists believe that more burials should have been found. Furthermore, four of the burials attributed by Mariette to the kingdom of Ramesses XI have since been retrodated. Scholars who favour changes to the standard Egyptian chronology, such as David Rohl, have argued that the dating of the twentieth dynasty of Egypt should be pushed some 300 years later on the basis of the Saqqara discovery. Most scholars rebut that it is far more likely that some burials of sacred bulls are waiting to be discovered and excavated