The earliest Egyptian tombs consisted simply of a hole in the ground, covered by a stone roof. Later tombs had low walls built to support the roof, usually sloping inwards slightly; these were called Mastabas. Building one Mastaba above another seemed a good idea at the time to emphasise the importance of its owner and allow for family burials. The five-story "step pyramid" of Zoser at Saqqara was the first such construction, and is believed to be the oldest stone building in the world. The smooth-sided pyramids of Giza were a natural progression from this concept.