It's one of the world's greatest mysteries. How did the ancient Egyptians build the pyramids? A big ramp, a big crane, aliens? Well, scientists think they've just solved that question. Researchers from the University of North Carolina, Wilmington, say 31 pyramids, including the world famous Giza Pyramids, built more than 4,000 years ago, are likely to have been made along a long lost ancient branch of the River Nile, which is now hidden under desert and farmland. This has been a theory for a long time, but only now have scientists found the location of the extinct river and its size. Using radar satellite imagery, scientists were able to map the river branch and find hidden features, including buried rivers and ancient structures which flowed alongside the pyramids. They think it was about 39 miles long and up to 700 meters wide, and was buried because of sandstorms and a major drought thousands of years ago. Scientists say this discovery helps explain how so many pyramids in the area were made and how the heavy blocks were transported using the river rather than human labor.