Professor Pierre Prévost, in 1789, gave an accurate explanation of the formation of the ice in Chaux-les-Passavant. He says: "Weighing carefully the local circumstances, one discovers in truth a few causes of permanent cold. But these causes seem rather suited to keep up a great freshness or to diminish the heat of summer, than to produce a cold such as that which reigns in the cavern. First of all, big trees throw shade over the entrance; it is, I was told, forbidden under severe penalties to cut down any of them, for fear of depriving the grotto of a necessary shelter. In the second place, this entrance is situated almost due north, leaning a little to the east, which is the coolest exposure one can choose, and the one most suited to help the effect of the icy winds which blow from that quarter. Finally the slope is steep and the grotto deep and covered with a thick vault. These three conditions united constitute, as it seems to me, a very good _ice house_; by which I mean a reservoir fit to preserve during the summer, the ice which may bank up in winter. "But how does this ice bank up? One knows that the outside waters above form on the roof, during the winter, long drops and stalactites of ice. These icicles, which hang down and increase constantly by the drip from the same source which formed them, fall at last, carried away by their own weight, and form so many centres, around which freeze the waters with which the floor of the grotto is always inundated. At the same time, the blowing of the north wind accumulates snow at the base of the slope, which is uncovered in part and exposed above to all the vicissitudes of the weather. Thus during the winter is formed an irregular heap of ice and snow, which the first heats of spring begin to make run, but which the heats of summer cannot finish dissolving. The winter following has therefore even more facility to augment the mass of these ice pyramids, which have resisted until the fall. And if men did not work at diminishing it, it might happen that it would fill the entire cavern at last to a great height. "I am therefore strongly inclined to think that the process of nature is here precisely similar to that of art; that without any especial cause of cold, the natural glacière of Besançon conserves in the moderate temperature of deep caverns, the heaps of snow and ice which the winds and the outside waters accumulate there during the winter; and that the melting of these snows and of these accumulated ices forms little by little the ice floor, scattered over with blocks and pyramids, which one observes there during the summer."