This is a work of speculation about a future half-a-million years to come. How will our species have changed? Or will we have made it at all, and if not, what kind of sentient minds might replace us and why?
First, a few words of caution: Wright's original novel, "The Amphibians," ended abruptly, without resolution to the main goal, which was the search for missing time travelers, and never explained the fates of the main characters. In the sequel, sometimes called "The Dwellers," but also referred to as "The World Below," we finally get the unanswered questions. It reads as a direct continuation of the original story, so a reader unfamiliar with "The Amphibians" would be hopelessly lost here, because there is absolutely no exposition to reintroduce you to the ongoing plot or characters. In some editions, however, both novels are combined to make a full and complete story arc, also titled "The World Below," so be wary of what version you are getting before you begin.
The other important thing to know before you start is that this is not a piece of pulp sci-fi, as some reviewers have wrongfully assumed. If you go into this novel thinking this will be a space opera, or H. G. Wells' "Time Machine," you will be disappointed. This is written as an allegory in the same vain as Dante's Comedia.
"The World Below," at least as a sequel, is a bit weaker than "The Amphibians," mostly because the plot is much more frequently interrupted by long philosophical dialogues between the two main protagonists. Even in the midst of danger, they pause and ramble on about how they each perceive and interpret events differently. It can really pull the casual reader out of their suspension of disbelief. But I think the author meant for these long exchanges to be occurring instantaneously between two telepathic characters, so that all of the information in the dialogues really were not meant to be taking place for hours while they are supposed to be fighting monsters. Still, the dialogues do get tedious, and the novel is much more preachy for it, rather than using allegory and good story-telling to unravel the author's intent.
Nevertheless, the dialogue does contain some mind-bending examples of how the same experience can be interpreted by two different people. It is the very essence of modern dialectical behavioral therapy. The main human character does change and grow as a result of realizing how his automatic assumptions, so normal in his culture, are really alien to his amphibian counterpart. Rather than rejecting the thoughts and behaviors of the strange world around him as "other," he realizes that his own ideas are completely biased by his own limited experience. He evolves as his outlook expands, and this is the real beauty of this journey, as opposed to the common "Us vs. The Alien Others" popularized in fiction from the 30s through the 50s, especially in America.
Speaking of aliens, we are not introduced to the variety of strange creatures in this story as in the original, though I do love the giant amphibious hippo-like monsters that feature as a minor threat in this tale. Also, the Dwellers themselves are quite terrifying, yet are made strangely and eerily sympathetic.
In summary, "The World Below," when together with "The Amphibians" as a solid whole, is a brilliant and underappreciated work of science fiction that I hope more modern readers rediscover and discuss.
Fun fact, the prolific sci-fi author Brian Stableford completed Wright's intended third part of the story called "The World Beyond." I will happily review that entry when I get my hands on it.