Looking underground from outer space was first demonstrated in 1981.

"It was just a fluke," recalls JPL Director Dr. Charles Elachi, who was the principal investigator of the Shuttle Imaging Radar-A (in 1981).
The radar onboard the shuttle was comprised of a single-frequency, single-polarization antenna capable of acquiring imagery at only one angle. Yet, the results were proof that certain radar frequencies could actually take images from as deep as 3 meters (9 feet) below the sand.

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