The Great Pyramid of Giza is a defining symbol of Egypt and the last of the ancient Seven Wonders. It was constructed over 20 years, concluding around 2560 BC, initially standing at 481 ft (146 m) tall.

It’s believed that this incredible building was built as a tomb for the Fourth dynasty Egyptian Pharaoh Khufu, also known as Cheops. Most of what we know about the pyramid comes from the Greek historian Herodotus, who visited in 440 BC.

We know that Egyptians traveled to the Red Sea during Khufu’s reign, evidence for this claim. It wasn’t uncommon for Pharaohs to claim they had “opened up” or “filled” waterways, which Herodotus believed was the case with the canal that connected the Nile River to the Red Sea.

On his journey there, he noted, “There is no country that possesses so many wonders, nor any that has several such works which defy description.” The Great Pyramid was also one of these many incomprehensible structures.

The entire structure is around 2.3 million stone blocks, with the heaviest weighing upwards of 15 tons. This construction is believed to have taken place during an era when tools were limited, making it all the more impressive that these ancient people would have built something so grandiose without modern technology.

While researchers have a good deal of information on the pyramid, there are still many mysteries surrounding it. For example, when you look at the Great Pyramid from above, it looks like a four-sided triangular structure. However, if you look at it from ground level, one side is noticeably longer than the others, and no one knows why.

Archeologists have found no evidence of tunnels or chambers within the pyramid, and Egyptologists still debate whether Khufu lies inside. Some believe a second burial chamber, possibly containing the Pharaoh’s mother, was a queen before him.

Regardless of where Khufu is, we know how he got here: an ancient ramp built next to the structure that spiraled upward. It had a steep grade of around 50 degrees and is estimated to be about 480 feet (146 meters) long.

Due to the pyramid’s design, some say it also served as an anchor for attracting extra-terrestrial crafts by acting as a giant antenna. Today, we know this theory as “Ancient Alien Astronaut.”