The passage twisted round a few turns, and then began to descend. It went steadily down for a long while before it became level once again. The air grew hot and stifling, but it was not foul, and at times they felt currents of cooler air upon their faces, issuing from half-guessed openings in the walls. There were many of these. In the pale ray of the wizard's staff, Frodo caught glimpses of stairs and arches, and of other passages and tunnels, sloping up, or running steeply down, or opening blankly dark on either side. It was bewildering beyond hope of remembering.
--"A Journey in the Dark" The Fellowship of the Ring

Artist: Johan
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