A volcano in the midst of the Rub' al Khali! And below me, as I stood on that hill-top transfixed, lay the twin craters, whose black walls stood up gauntly above the encroaching sand like the battlements and bastions of some great castle. These craters were respectively about 100 and 50 yards in diameter, sunken in the middle but half choked with sand, while inside and outside their walls lay what I took to be lava in great circles where it seemed to have flowed out from the fiery furnace. Further examination revealed the fact that there were three similar craters close by, though these were surmounted by hills of sand and recognizable only by reason of the fringe of blackened slag round their edges.