"A sandy plain, extending as far as the eye could reach," he says, "and upon which arose innumerable hills in the semblance of waves,--such was the scene presented to my gaze. Not the least trace of vegetation was perceptible; not a bird interrupted with its song the tomb-like silence which prevailed around the graves of the Sabean army. I remarked three tracts distinguished by a dazzling whiteness. 'Yonder are the abysses,' said the Bedouins; 'they are inhabited by the spirits who have covered with this deceitful sand the treasures intrusted to their charge. He who dares approach them will assuredly be dragged down under the sand! Do not venture there!' "Naturally, I paid no attention to this counsel; on the contrary, I demanded to be guided towards them, according to agreement. Two hours were consumed by our camels in reaching the bottom of the plateau, where we arrived at sunset, taking up our quarters for the night on the lee side of two enormous rocks. On the following day I insisted that the Bedouins should guide me over these tracts. My trouble was in vain; fear rendered them unable to utter a word. Furnished with a plumb-lead weighing about a pound and a quarter, to which was attached a rope nearly 350 yards in length, I accomplished this dangerous enterprise. I occupied thirty-six minutes in reaching the first abyss; it was thirty-six feet long by twenty-six feet broad, and formed an inclined plain towards the centre, about six feet deep, which I attributed to the action of the wind. I approached at first with the utmost precaution, in order to examine the sand, and found it to be almost impalpable. I cast my plumb-lead as far as I could; it disappeared immediately; however, the rapidity with which the rope shortened gradually diminished; in five minutes, it had wholly disappeared." Baron de Wrede has made no attempt to account for this strange phenomenon, which is not, I may add, peculiar to Arabia. The late Doctor Cloquet, who for many years acted as chief physician to the Shah of Persia, relates that he had seen similar gulfs in the great Salt Desert, which he considered to occupy the place of lakes suddenly vanished. This hypothesis is certainly admissible, and perhaps very probable; but while in some degree explaining the existence of these abysses of sand, it raises fresh questions which are by no means easily answered; for instance, why have these lakes disappeared, and why have they been replaced by this impalpable and incoherent dust in which heavy bodies sink as in a void? Consider, moreover, the remarks made by Doctor Cloquet in a letter addressed in 1851 to the Academy of Medicine at Paris:--