Very interesting
.
They've been using remote sensing technology for years to peer under the deserts in Africa, and in other parts of the world. They used to fly X-band radar in the Shuttle cargo bay for that purpose. In the late 80's-early 90's, they found complete evidence for a river under the Sahara that was both larger and longer than the Nile, flowing west to east. Many of the oases and some wadis in the desert were surface remnants of the old river system.