What lies under most of modern Cairo's suburbs?

The necropolis of Memphis, the capital of Old Kingdom Egypt, which ran for tens of kilometres along the west bank of the Nile. Giza is the northern end of it; Saqqara, with the step pyramid of Djoser - the first large stone building anyone made - is further south, and the whole strip is a cemetery city that was in use for three thousand years.

The city of the living is largely gone. Memphis itself was built of mud brick and has almost entirely dissolved back into the floodplain, while the tombs were built of stone on the desert edge, deliberately above the flood line. That is why Egypt is known through its dead: the durable buildings were the ones meant to last for ever, and the ordinary ones were not.

Cairo has now grown out over it. Suburbs press against the Giza plateau, and the modern city's edge is visible in photographs taken from the pyramids in the direction nobody points the camera. Cairo also contains a genuine City of the Dead - a several-kilometre stretch of Mamluk-era tomb complexes in which a substantial population has lived for generations, among and inside the mausoleums.