People are always curious about what lies beneath the sand, whether it be due to archaeologists or simply some boys on the beach digging a hole that looks like an asteroid hit it.

Naturally, the solution varies based on the type of sand. Sand is produced by the ceaseless churning of the sea as it grinds up nearby rocks over thousands or millions of years. Sandstone, a sedimentary rock formed by the pressure of the layers above, can be found beneath the ocean sediment. If you dig through this, you might uncover fossils or oil (which, by the way, is not made of dinosaurs). Fossils are generated when minerals seep into an organism and transform it to stone.

Sand can also be created on land from rocks that have been weathered by rivers and other factors. The Sahara desert is one place where it is (kind of) present. The Sahara is well-known for its huge sand dunes, yet unlike most deserts, it is primarily composed of other materials.

What then lurks below the sand dunes of the Sahara? As it turns out, many intriguing discoveries. The Sahara wasn't always the parched, desolate region that it is now. According to a recent study, the change in monsoon seasons brought on by the tilt of the Earth's axis causes it to turn green around every 21,000 years.

The Nubian Sandstone Aquifer System, the largest fossil water aquifer system in the world, is just one of the desert's many surprises. One was a prehistoric mega-lake that was discovered back in 2010. Researchers have discovered evidence that the Nile flooded the eastern Saharan desert some 250,000 years ago, spanning an area of more than 108,800 square kilometers (42,000 square miles) and covering fish fossils discovered 400 kilometers (250 miles) west of the Nile.

"These lakes could have originated from overflow of the Nile through Wadi Tushka, the low point on the west bank of the Nile. An overflow origin is consistent with recent hypotheses on the origin of the Nile and its integration with drainage from Central Africa," the team wrote in their study, adding that "the topographic data add to the growing evidence for numerous early and middle Pleistocene lakes across North Africa that could have supported human migration patterns."

The scientists concluded that Paleolithic people most likely established towns close to the old mega-lake's water supply.

Despite how hostile the Sahara may appear, people have lived there for ages, and over time, settlements and forts from long-lost civilizations have been found there. You won't be surprised to find that our good friend sandstone is once again beneath all of this. Bedrock and dried-out mud can be found if you delve deep enough through the sandstone.