Tony Stewart remains in an undisclosed Southern California hospital, recovering from back injuries suffered Sunday when he crashed a sand dune car in the desert.

So what exactly is a sand dune car? It's an off-road vehicle custom-built with a tube frame, powerful V-8 motor, huge tires and all-wheel drive for traversing desert hills. And they are very popular.

A number of NASCAR racers own or have driven sand dune cars for fun in their off time. The accompanying video from the 2015 NASCAR Western Swing shows Greg Biffle demonstrating the capabilities of these all-terrain vehicles. Biffle isn't alone; the Busch brothers, Casey Mears and others have taken to the dunes as well.

Stewart-Haas Racing officials have said they do not expect to have an update on Stewart's condition until Thursday afternoon. They have not yet indicated how long they expect the three-time NASCAR Sprint Cup Series champion to be out of action.

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