It was at the same time, between 1883 and 1885, that I made my first investigation in the Causses of Lozère, Aveyron, and the adjoining departments of France, the results of which were to reveal for the first time to the public, and even to geographers, the picturesque beauties, then unknown, and now becoming the fashion, of the gorges of the Tarn, Jenta, and Dourbie, the rocks of Montpelier le Vieux, etc. In my excursions over the plateaus of the Causses I frequently met, at the level of the surface, open, dark holes, and mouths of vertical wells--_avens_--the depths of which no one had ever looked into, unsoundable, they said, which the peasants naturally took to be real mouths of hell. Recollecting what I had admired at Adelsberg and in various caves of the Pyrenees, I guessed these avens might also be doorways to subterranean splendors and scientific treasures. So I began in 1888 the methodical exploration of the unexamined natural cavities of my own land first, and then of other countries of Europe; and since then I have devoted several weeks every year to this work. These pits are simply horizontal holes opening upon the surface of the ground, of very different forms and dimensions. Herdsmen are very careful not to let their cattle go too near them, for they sometimes fall in. The diameter of these pits varies from a few inches to several hundred yards, and they are sometimes more than six hundred feet deep. It is not easy to go down into them, especially when they are on high levels away from habitations and roads. In such cases a considerable apparatus of ropes, rope ladders, telephone, portable boat, tent, etc., has to be taken along. The first measurement with the sounding line gives the depth only of the first pit--and there are often several succeeding one another. A rope ladder long enough to reach the bottom is then let down, and the man who descends has a rope tied about him for additional security, which is held by the people above. A great many pits are narrower at the top than lower down, forming something like a reversed speaking trumpet, so that the explorer finds it very difficult to make himself heard at the top; hence I have adopted the practice of taking a telephone along. The interior shapes of the pits are very diverse. The narrower ones are easiest to go down, because they permit one partly to support himself against their walls. The wider ones leave him hanging loose, in a position which he feels to be very precarious. When there is a second or third pit, and we have not ladders enough, we have to trust ourselves to a simple rope with a board fastened at the end of it for a seat. The _gouffre_ of Vigne Close, in Ardèche, which is about six hundred feet deep, has five successive pits, and its complete exploration required three days. The bottom of the pit may be a simple cleft in the rock, or an immense cathedral-like chamber; as at Rabanel, near Ganges, and Hérault, the deepest abyss in France, the