There are hells everywhere. They are under the mountains and hills and cliffs and under the plains and valleys. The openings or gates to the hells that are under the mountains and hills and cliffs look at first sight like crevices or fissures in the rocks. Some of them are quite broad and open, some narrow and confined, full of rough places. All of them seem dim and gloomy when you look in, although the hellish spirits who live there have the kind of illumination you get from glowing coals. Their eyes are adjusted to the reception of this kind of light. This is because when they were living in the world they were in darkness about divine truths owing to their denial of them. They were in a kind of light as to their false convictions because they affirmed them, which gave their eyesight the form it has. This is also why heaven’s light is darkness to them, so when they come out of their caves, they cannot see anything. This makes it abundantly clear that we come into heaven’s light to the extent that we have acknowledged the Divine and affirmed within ourselves the values of heaven and the church. We come into the darkness of hell to the extent that we have denied the Divine and affirmed within ourselves values contrary to those of heaven and the church.
The entrances or gates to the hells that are under the plains and valleys have different forms. Some of them are like the ones under the mountains, hills, and cliffs; some of them are like caves and caverns; some are like large chasms and quagmires, some like swamps; and some like stagnant ponds. All of them are covered over and are visible only when evil spirits are being cast in there from the world of spirits. When they are opened, something breathes out like the smoky fire we see in the air from conflagrations, or like flame without smoke, or like the kind of soot that comes from a hot chimney, or like a dark storm cloud. I have heard that evil spirits neither see nor feel this because when they are in it, they are in their element and therefore in the delight of their life. This is because these appearances correspond to the evils and distortions they are absorbed in, the fire corresponding to their hatred and vengefulness and the smoke and soot to their consequent distortions, the flame to the evils of their self-love, and the dark storm cloud to the distortions that follow from it.
from Heaven and Hell, Sections 584-585