The pyramids in Egypt were built as royal tombs for the pharaohs—first along the edges of cliffs as low rectangular structures called mastabas, then as tall, four-sided pyramids. The three structures most of us associate with pyramids were built at Giza, near Cairo, about 2500 B.C.E. The largest of these, the great pyramid of the pharaoh Cheops, measures 481 feet (147 meters) high. But there are no intricate pathways in this pyramid; it is merely a pile of limestone blocks weighing between 3 and 15 tons.

Why did the Egyptians choose the pyramid form? Historians know that the Egyptian Sun god, with its rays reaching for the Sun, was represented by pyramid-shaped stones, or ben-bens. And because the Egyptians worshipped the Sun as their chief god, the pyramids are thought to be huge renditions of the ben-bens. After a pharaoh died, the pyramids would be symbolic of the pharaoh ascending the Sun’s rays to join his Sun god.

Some historians also believe that the pyramids may have had some (still hidden) numerical significance. In particular, some believe the ratio of the pyramid’s perimeter to its height times two, or P/(2 × H), gives a close approximation to the value of pi (π); another claim is that the slopes of the pyramids’ sides were also expressions of pi.