The ancient pyramids of Egypt are still a mystery today. No one knows exactly how the Egyptians built them. The biggest pyramid is the great Pyramid of Giza. The Egyptians built it in about 2550 BCE. The workers used 2.5 millian limestone blocks. The great pyramid convers 123 acres (50 hectares or 500,000 square meters) It's 480 feet (146 meters) high. The sides are 755 feet (230 meters) long.