Farther on the eye discerned their massive outline sharply defined against the hazy distant horizon.
It was quite an ocean, with the irregular shores of earth, but desert and frightfully wild in appearance.
If my eyes were able to range afar over this great sea, it was because a peculiar light brought to view every detail of it. It was not the light of the sun, with his dazzling shafts of brightness and the splendour of his rays; nor was it the pale and uncertain shimmer of the moonbeams, the dim reflection of a nobler body of light. No; the illuminating power of this light, its trembling diffusiveness, its bright, clear whiteness, and its low temperature, showed that it must be of electric origin. It was like an aurora borealis, a continuous cosmical phenomenon, filling a cavern of sufficient extent to contain an ocean.
The vault that spanned the space above, the sky, if it could be called so, seemed composed of vast plains of cloud, shifting and variable vapours, which by their condensation must at certain times fall in torrents of rain. I should have thought that under so powerful a pressure of the atmosphere there could be no evaporation; and yet, under a law unknown to me, there were broad tracts of vapour suspended in the air. But then 'the weather was fine.' The play of the electric light produced singular effects upon the upper strata of cloud. Deep shadows reposed upon their lower wreaths; and often, between two separated fields of cloud, there glided down a ray of unspeakable lustre. But it was not solar light, and there was no heat. The general effect was sad, supremely melancholy. Instead of the shining firmament, spangled with its innumerable stars, shining singly or in clusters, I felt that all these subdued and shaded fights were ribbed in by vast walls of granite, which seemed to overpower me with their weight, and that all this space, great as it was, would not be enough for the march of the humblest of satellites.
Then I remembered the theory of an English captain, who likened the earth to a vast hollow sphere, in the interior of which the air became luminous because of the vast pressure that weighed upon it; while two stars, Pluto and Proserpine, rolled within upon the circuit of their mysterious orbits.
We were in reality shut up inside an immeasurable excavation. Its width could not be estimated, since the shore ran widening as far as eye could reach, nor could its length, for the dim horizon bounded the new. As for its height, it must have been several leagues. Where this vault rested upon its granite base no eye could tell; but there was a cloud hanging far above, the height of which we estimated at 12,000 feet, a greater height than that of any terrestrial vapour, and no doubt due to the great density of the air.
The word cavern does not convey any idea of this immense space; words of human tongue are inadequate to describe the discoveries of him who ventures into the deep abysses of earth.
Besides I could not tell upon what geological theory to account for the existence of such an excavation. Had the cooling of the globe produced it? I knew of celebrated caverns from the descriptions of travellers, but had never heard of any of such dimensions as this.
If the grotto of Guachara, in Colombia, visited by Humboldt, had not given up the whole of the secret of its depth to the philosopher, who investigated it to the depth of 2,500 feet, it probably did not extend much farther. The immense mammoth cave in Kentucky is of gigantic proportions, since its vaulted roof rises five hundred feet  above the level of an unfathomable lake and travellers have explored its ramifications to the extent of forty miles. But what were these cavities compared to that in which I stood with wonder and admiration, with its sky of luminous vapours, its bursts of electric light, and a vast sea filling its bed? My imagination fell powerless before such immensity.
I gazed upon these wonders in silence. Words failed me to express my feelings. I felt as if I was in some distant planet Uranus or Neptune - and in the presence of phenomena of which my terrestrial experience gave me no cognisance.