Reviewer Bart Higgins noted the similarity between Tolkien's Mount Doom and the volcanic caverns under Venus depicted in Perelandra by Tolkien's friend C.S. Lewis. In both cases, the book reaches a cataclysmic scene of a struggle in a cavern, ending with an evil being falling into volcanic fire and being consumed. Since it is well known that Tolkien and Lewis frequently shared with each other their respective works in progress, it is reasonable to assume that the resemblance is no coincidence.