For our purposes, the most significant and unusual property of microwave radiation is that longer wavelengths have the ability to pass through fine, dry sand. No one had ever worked through the physics to predict how microwave radiation would behave under these conditions, so NASA scientists were completely surprised--and not a little perplexed--by the first radar images of the Sahara Desert, which were captured by an early radar imaging system called SIR-A. Since then a great deal of analysis has been done to better understand the interaction of microwave radiation with sand.
On the left is a (visible-light) photograph taken through the window of the Space Shuttle of the Sahara Desert. On the right is a Landsat image (also visible light) with a swath of radar imagery overlaid. Note the spidery dark lines in the radar image: these are underground rivers which show up on the radar because the radar radiation has passed through the sand covering these channels. The underground rivers look like they're sitting right on the surface.