_MrAvocadoMan_: The base of dunes is not flat, but generally follows the contours of the paleotopography. A large portion of what is now the Sahara consisted of four “megalakes”. This is what the interior of a sand dune looks like with a long-range ground penetrating radar: http://imgur.com/UrQBsUO. The base of this Libyan dune is caliche (evaporates) but is not flat.”
philip1201: Sandstone (sand compressed over millions of years to become rock) or metamorphic rock (magma which gradually came out through a [crack in the earth](https://en.wikipedia.org/wiki/Divergent_boundary) billions of years ago, and became solid slowly while in chemical interaction with the magma below, rather than suddenly in a volcano), depending on where you look.
ggchappell: Most of the Sahara (around 80%) is not covered with sand. There are lots of mountains, rocky plateaus, and generally rugged terrain. Take a look at it in Google Earth; seriously — it’s interesting terrain.
That said, there are some areas of the Sahara that are entirely sand. If such an area is large, it is called an *erg*; a smaller sand region is a *dune field*. Ergs & dune fields generally form in areas that used to be water: rivers and lakes. So what’s underneath the sand today are dried up watercourses or lakebeds.
Emphos: Minecraft knowledge says sandstone, then some like in most other places
torbjorn_bradda: Probably just compressed sandstone, but in [my imagination there’s more than a few undiscovered cities buried there!](https://en.m.wikipedia.org/wiki/Kaymakli_Underground_City)
rrpjdisc: Basically a rock layer.
Just wanted to give a simple answer for anyone else that was initially confused by responses to this question.
MarzipanFairy: A statue of Ozymandias.
CoinsOnTheMoon: Follow up question, where is all the sand blowing?
thehaga: Shit from the 250k year old megalake they discovered, some weird [structures](http://www.ancientpages.com/2015/05/30/mysterious-ancient-structures-hidden-sand-sahara-desert-re-write-history-ancient-egypt/), and bunch of other shit depending on where you look – it’s a massive area and they haven’t dug everything up yet so could be all kinds of stuff.
JayWrenFlew: Packed sand
BusyNess101: You don’t wanna know what else is down there
sebastiaankas: Mostly lava
RyanTheCynic: Contrary to popular belief, it’s sand all the way down.
Fidelian: Besides all the sandwiches there?