No matter how much we discover about the famed Great Pyramids of Giza in Egypt, there always seems to be an air of mystery surrounding these structures. Constructed roughly 4,500 years ago, these massive relics of the Old Kingdom era have survived seemingly everything.

The sheer mass of the pyramids alone is staggering — the largest one stands 480-feet high and is made up of 2.3 million stone blocks. To this day, scientists still speculate about the construction methods used by Egyptians to build these ancient architectural wonders.

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Because Egyptian Pharaoh’s believed that they would become gods in the afterlife, they created these pyramids as ornate tombs filled with everything they would need to thrive in the next world after death. Production of the first pyramid was started by Pharaoh Khufu around 2550 B.C. His son, Pharaoh Khafre built the second, slightly smaller pyramid around 2520 B.C., in addition to the enigmatic limestone Sphinx that is thought to stand guard over this tomb. The third temple was built in 2490 B.C. by Pharaoh Menkaure, and is significantly smaller than the first two structures, but features a more intricate interior temple.

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It is said that these stunning structures hold the secrets of the ancient Egyptian pharaohs. While scientists have been able to discover much about the different rooms and chambers within these pyramids, there are still questions left unanswered. Here are some interesting facts about these ancient and mysterious marvels.

The Great Pyramid of Giza is the last standing ‘wonder’ of the Seven Wonders of the Ancient World.

The Seven Wonders of the Ancient World are as follows: The Great Pyramid of Giza, the Hanging Gardens of Babylon, Temple Artemis at Ephesus, the Statue of Zeus at Olympia, the Mausoleum at Halicarnassus, Colossus of Rhodes, and the Lighthouse of Alexandria. Six of the seven of these ancient structures were destroyed either by natural causes, like earthquakes, or by human plundering—except for the Great Pyramid of Giza. This structure has survived a lot.

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The Great Pyramid of Giza was the tallest man-made structure on earth for 3,871 years.

From its time of construction until 1311, when the Lincoln Cathedral in England was completed, the Great Pyramid stood taller than any other man-made structure across the globe. Today, the Burj Khalifa in Dubai is the world’s tallest manmade structure, standing at 2,722 feet.

It’s thought that as few as 20,000 or as many as 100,000 workers were involved in the construction of the pyramids.

This assumption made by historians and scientists is based around two ideas: the first being that the ancient Greek historian Herodotus claimed that construction took 20 years and 100,000 men, which is believable because these agricultural workers were likely only building when the Nile River flooded, leaving less work to be done in the fields, and more hands ready for the pyramid. However, archeologists later found evidence in the 20th century that as few as 20,000 workers could have been permanently (rather than seasonally) building the Great Pyramid.

In the 12th century, Kurdish ruler al-Malek al-Aziz Othman ben Yusuf attempted to destroy one of the pyramids, but only successfully damaged the smallest, leaving a vertical gash on the north face.

This attempted destruction was unsuccessful, as the task was just too big (no kidding), proving to be incredibly expensive and time consuming. Menkaure’s pyramid suffered deep gash, but is otherwise intact to this day.

The Orion Correlation Theory

Introduced in 1983 by Robert Bauval, this theory makes the claim that the three Great Pyramids of Giza are positioned in alignment with the three main stars of the constellation Orion’s Belt — and that this orientation was intentional. While it seems possible, this theory has received a good amount of criticism and rebuttal.

Want to read about more secrets? Check out the coolest secrets of the Hollywood sign in Los Angeles.