Spoiler alert: This post is part of an in-depth discussion of Perelandra by C. S. Lewis, which will inevitably reveal key plot points. I recommend you support your local bookseller or public library, and read the book first.
Ransom's battle with Weston, the Un-Man who came to Perelandra to corrupt it, took place over land, on the sea, and then deep underground, before Ransom finally won, bashing Weston's head with a stone and throwing him into the subterranean fires. Exhausted, he slept.
When he awakens, Ransom starts exploring the caverns, trying to find his way out of them. The sights he sees are indistinct and surreal, as though he is delirious from fatigue and unsure what is real. He sees many strange things underground: a vast, cathedral-like vault with two giant thrones, clearly not intended for Pereldandra's human inhabitants. Deep below him he sees more insectoid creatures like the one that had emerged from the depths with Weston, but there are harnessed to a cart carrying an upright, cloaked form.
After falling into an underground river and being whisked by the strong current through what appears to be a giant ice cave, he finds himself deposited on the edge of a large pool, in daylight. Ranson guesses himself to be atop a high mountain. He finds fruit to eat, and again falls asleep. On this ledge, he rests and recuperates from his fight for what seems to him to be only a day or so, but could have easily been weeks—he is unaware of the passage of time.