Saqqara is an immense necropolis (cemetery) just south of Cairo and west of the ancient city of Memphis of which very little remains. Used as a burial ground for thousands of years, Saqqara hides its secrets well under desert sands. Despite virtually continuous excavations for some two centuries, much of the area remains to be excavated. The site stretches six kilometres from north to south and more than 1.5 kilometres across at its widest point. The famous Step Pyramid is on the right with the ruined pyramid of the 5th Dynasty pharaoh Userkaf on the far left. The site's best-known feature is the Step Pyramid, the world's oldest major stone structure. It was built in the 3rd Dynasty (around 2630 BC) for King Djoser and its construction was overseen by his vizier Imhotep. The Step Pyramid of Zoser. All over Saqqara can be found tombs of different periods. Those open to the public date to the Old Kingdom. Around the northern-most of Saqqara's pyramids is that of the 6th Dynasty pharaoh Teti. Adjacent to the pyramid are the mastabas (free-standing tombs of earlier periods) of his officials, some of whom had marvellous reliefs created for themselves. One of Saqqara's most famous archaeological sites is the Serapeum which was discovered by Auguste Mariette in 1851. Its rockcut corridors and burial chambers were excavated for the Apis bulls which were sacred to god Ptah. The corridors form a virtual underground extending for hundreds of metres. The stone sarcophagi weigh as much as 70 tonnes and average some 4 metres in length and 3.3 metres in height. Twenty chambers still contain sarcophagi. The Serapeum was in use from the New Kingdom down to the Graeco-Roman period.