As one of the Seven Wonders of the  World, the Great Pyramid of Giza has fascinated people for a long time.

Probably pretty much since it was built. And in that time, we’ve learned a lot  about it and other pyramids around Giza.

We know that they were constructed  between roughly 2575 and 2465 BCE, and we know they were built  to be tombs and temples. But one thing we didn’t know is  why they were built where they are. Because those things are in  the middle of the desert, kilometers away from where we’d  expect them along the Nile River.

After centuries of study, though, we  may have finally solved that mystery. And it turns out the answer may have been hiding just below ground the whole time. [♪ INTRO] So to start off, the Giza  Pyramid Complex is actually way more than just the Great Pyramid. There are also two other main  pyramids, plus the Sphinx, plus several smaller pyramids and temples.

The whole complex was built about  7 kilometers west of the Nile and about 10 kilometers from Memphis,  the capital of Egypt at the time. So pretty much of out of the way when you don’t have a dune buggy to get around on. And the tools and materials used to  build the pyramids came from all over, including copper from the Sinai  peninsula, timber from Lebanon, and granite for the exterior from  the city of Aswan, up the river.

Papyri from the Sinai peninsula tell  us that the Egyptians used barges to transport heavy building materials down the Nile, and then possibly created artificial waterways to get the supplies from the  Nile to the pyramid site. But that would still mean making  over 7 kilometers of waterway, so the question remains why they picked  that exact site in the first place. Like, instead of building canals  leading to an artificial lake, it would have been a lot easier to  just…pick somewhere closer to the Nile.

Well, it turns out that they were a lot  closer to running water than we thought. Using radar satellites,  researchers have been able to get a picture of what was under  the surface of the desert. And they were able to see these tiny  little depressions, which were covered up by nearby farmland, which is why  they’d been so hard to spot before.

When the researchers took  samples of the earth from more than a dozen meters below the  surface, they found sand at the bottom. And yeah, that doesn’t sound like a very big deal. Like, of course there’s sand.

It’s a desert. But sand is normally only the top  layer of a desert environment, and so when you find it lower down, that usually indicates that there used  to be running water there. Putting together the channels they  saw on the satellite imagery with the discovery of sand beneath the  surface led the researchers to conclude that thousands of years ago, a branch  of the Nile flowed through the area, parallel to the main river, but  several kilometers to the west.

They’re calling this now-dried  up branch the Ahramat branch, which is Arabic for “Pyramid branch”. Which is pretty fitting, since aside  from Giza, there are 31 pyramids located along the 64 kilometer stretch of the  former river that they were able to track. Thanks to Arizona State University’s  College of Global Futures for supporting this SciShow video!

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The pyramids on the Ahramat  branch were built over the thousand year period between 2600 and 1600 BCE, suggesting that this was a significant  Egyptian waterway for quite a while. But now, just a few thousand years later, there’s barely a trace of the Ahramat branch left. So what happened to it?

It’s likely that the disappearance  of the Ahramat is related to larger changes in the Nile River  Valley over the last 14,000 years. Back then, the eastern Sahara Desert  was a lot wetter than it is today, thanks to stuff like glacial  melt and sea level rise. There were large river systems and lakes,  and the amount of rainfall actually made it a savannah or  grassland instead of a desert.

It’s even known as the African Humid Period. At that time, there was so much water  that the Nile River Valley was a soupy, swampy mess that meant it  was pretty much inhospitable, which meant that there was  enough water that the Nile River would have easily formed other branches,  and nobody could actually live anywhere near the actual rivers because  there was too much swamp in the way. Somewhere between 6 and 10,000  years ago, things started to dry out enough to allow people to live a  little closer to the riverbanks.

And then about 5,000 years ago,  things shifted even more dramatically. Less rainfall allowed the sediment  to build up in the Nile floodplain, which meant there was a whole bunch  of dry farmland above the marshes. And the new dryness of the  Sahara pushed settlements even closer towards the Nile,  where there was still water.

But things were still wetter than they are now, allowing for those branches  of the Nile to keep thriving. Branches like the Ahramat. When and where settlements were  built tells us what was happening with the location and water  level of the Ahramat over time.

And tracking those settlements showed  that the river started to dry up, but also slowly shifted more towards the east. This change was likely due to a few factors,  including a shift in tectonic plates that tilted the ground so the  Ahramat was basically uphill from the main stretch of the Nile. And then that tilt, combined  with windblown sand and sediment building up along the river, caused  the Ahramat to finally run dry.

And just like that, the river  was hidden for millennia. Uncovering the Ahramat is  the first step in discovering more hidden aspects of Ancient Egypt. Because we know that people like  to build settlements along rivers, researchers think that following the  branch’s path will allow archaeologists to discover new historical  sites and better preserve them.

It also provides new information on the  cultural development, and interconnectivity, and the trade routes of Ancient Egypt,  because it means that many cities and towns were directly connected  by river, not just over land. Which just goes to show that  even after thousands of years, there’s still so much more  to learn about the pyramids. Who knows what other secrets are  buried in the sands of the Sahara? [♪ OUTRO]