When ancient architects completed construction on the Great Pyramid at Giza, they left behind the greatest riddle of the engineering world - how did builders lift limestone blocks weighing an average of two and a half tons, 480 feet up onto the top of the pyramid? For centuries, adventurers and Egyptologists have crawled through every passageway and chamber of the Pyramid, measuring and collecting data in an attempt to determine how it was built. For the first time, a revolutionary theory argues that the answer may be inside the pyramid. National Geographic follows renowned experts as they find surprising evidence to support this intriguing new theory.