I want to build a large, horizontal underground tunnel that spans my world, for quick travel. Unfortunately, if I tunnel under a desert all the sand falls down and I just make a giant hole wherever my tunnel is. I could put a layer of stone above the tunnel to support the sand, but I can't find a way to make space to put the stone without the sand falling.
One solution I have thought of is to dig out the entire desert, build my tunnel, and then put all the sand back, but that doesn't sound like fun at all.
EDIT: The solution I decided to go with is just dig below the desert. I am making my "subway" at a depth of "level" (0), so hopefully I won't have much trouble with sand. Thanks for the suggestions!