Even with the advantage of modern technology, replicating the building of Egypt ’s great pyramids would prove costly and time-consuming. Back in the days of the pharaohs, it took about 20 years and thousands of slaves to construct the Great Pyramid of Giza (a.k.a. the Pyramid of Khufu). The ancient marvel stands 756 feet long on each side, 481 feet high, and is composed of 2.3 million stones weighing nearly 3 tons each. To reconstruct the pyramid today, using trucks, cranes and helicopters, the project would still require 1,500 to 2,000 workers and cost about $5 billion, according to Jean-Pierre Houdin, a French architect who has helped create a virtual model of the ancient construction system. And it would take five years to complete.