They received their gold and silver from deep underground limestone mines that formed under the desert sand, which was dug up in massive quantities (sometimes 57 tons at a time!) to allow access to the caves. They usually spiraled down 21, 196 km (13, 170 miles), and about 2,000 workers and slaves were sent down at a time to recover the precious metals. These caverns were so extensive, however, that only one out of the 400 known caves was mined completely by the ancient Egyptians. Typically, the workers sent down would use spikes to mark their descent as to not get lost, but many simply failed to come back up once the ending gong, a large gong signaling the end of a shift, was rung. Also, many of the ropes that the workers used were worn out and flimsy. As they would wall-hop down, the rope would snap and they would plummet thousands of feet to their death. None of the bodies were ever recovered. Also, the threat of a cave in was always imminent. Many times the workers would over-mine a certain area and get buried under tons of rubble. It is estimated that a shocking 292 million workers were lost in the caves.