What is underneath the desert?

Dust.
Soil
Rocks.
Tree roots.
Underground animals.
Skeletons.
Water.
Lava.
2nd layer of the Earth.
3rd layer of the Earth.

[Mod says] This is in general folks. Responses must be helpful and on topic.

Here is a cross-section of the Earth showing the origin and layers for 2 deserts in the US.
Arizona Desert Museum

In Death Valley California some people think that there is gold under the desert.

You’re making the mistake of thinking that it is sand which defines “the desert”. A simple trip to the beach should demonstrate the fallacy of the assumption. Sand is much more the exception than the rule for most of the land designated as desert. It’s climate or lack of water that determines whether or not you’re standing in a desert.

One would think that somewhere under the desert IS water in some form(moisture) to enable some species of plantlife and reptiles to live?