The tremendous vault above our heads, the sky, so to speak, appeared to be composed of a conglomeration of nebulous vapors, in constant motion. I should originally have supposed that, under such an atmospheric pressure as must exist in that place, the evaporation of water could not really take place, and yet from the action of some physical law, which escaped my memory, there were heavy and dense clouds rolling along that mighty vault, partially concealing the roof. Electric currents produced astonishing play of light and shade in the distance, especially around the heavier clouds. Deep shadows were cast beneath, and then suddenly, between two clouds, there would come a ray of unusual beauty, and remarkable intensity. And yet it was not like the sun, for it gave no heat. The effect was sad and excruciatingly melancholy. Instead of a noble firmament of blue, studded with stars, there was above me a heavy roof of granite, which seemed to crush me. Gazing around, I began to think of the theory of the English captain who compared the earth to a vast hollow sphere in the interior of which the air is retained in a luminous state by means of atmospheric pressure, while two stars, Pluto and Proserpine, circled there in their mysterious orbits. After all, suppose the old fellow was right! In truth, we were imprisoned--bound as it were, in a vast excavation. Its width it was impossible to make out; the shore, on either hand, widening rapidly until lost to sight; while its length was equally uncertain. A haze on the distant horizon bounded our view. As to its height, we could see that it must be many miles to the roof. Looking upward, it was impossible to discover where the stupendous roof began. The lowest of the clouds must have been floating at an elevation of two thousand yards, a height greater than that of terrestrial vapors, which circumstance was doubtless owing to the extreme density of the air. I use the word "cavern" in order to give an idea of the place. I cannot describe its awful grandeur; human language fails to convey an idea of its savage sublimity. Whether this singular vacuum had or had not been caused by the sudden cooling of the earth when in a state of fusion, I could not say. I had read of most wonderful and gigantic caverns--but, none in any way like this. The great grotto of Guachara, in Colombia, visited by the learned Humboldt; the vast and partially explored Mammoth Cave in Kentucky--what were these holes in the earth to that in which I stood in speechless admiration! with its vapory clouds, its electric light, and the mighty ocean slumbering in its bosom! Imagination, not description, can alone give an idea of the splendor and vastness of the cave.