The Great Pyramid of Giza – the oldest and largest of the three pyramids in the Giza pyramid complex – has been at the center of conspiracy theories for decades as people have struggled to understand how it was constructed. Was it a calling card left behind by aliens? Did UFOs lend insight on how to construct it? Here’s what we now know about this historic structure that has mystified researchers and theorists.

Last Man Standing

Egypt’s Great Pyramid of Giza is not only the oldest of the Seven Wonders of the Ancient World, but the only one that has survived all the way to modern times. Constructed between 2560 and 2540 BCE, the pyramid was built for Pharaoh Kufu – the second pharaoh of the 4th Dynasty. It is the largest of all the tombs built in the ancient world and has towered over the West Bank of the Nile River, south of Cairo, for more than 4,000 years.

While the Great Pyramid was once the tallest man-made structure in the world – a title it held for thousands of years – it was eventually overtaken by the Lincoln Cathedral in 1311 CE. It once stood at 481 feet tall but has since shrunk down by about 25 feet as a result of erosion.

All That Glitters Wasn’t Always Gold

Despite the golden images we now often see of the pyramid, a modern documentary by historian Bettany Hughes claims that it was pure white when it was built. Citing a 4,500-year-old journal, Hughes has stated that it was constructed from polished white limestone – 2.6 million casing stones, in fact, which took about 20 years to turn into the pyramid we recognize from history books. The stones, which weighed in at 2.5 tons each, were carried for 15 miles along the Nile River. The final structure is estimated to weigh about 6.5 million tons. The pyramid no longer has that pure white appearance because its outer limestone shell was stripped away over the years, potentially for use in other building projects.