The second part was published separately by Galaxy Science Fiction Novels in 1951 and was also titled The Worlds Below.

The novel concerns a man who travels 500,000 years into the future with the aid of a time machine.

Boucher and McComas praised the 1949 edition, citing its "sociological criticism, spiritual stimulation and satire of high order.

"[1] In 1950 L. Sprague de Camp characterized it as "one of the most remarkable science-fiction stories ever written," saying that "For atmosphere the book is absolutely unsurpassed, and for exciting action it rates high."

de Camp, however, also faulted the novel's "long philosophical arguments" in dialogue between its hero and his non-human companion, particularly their "straw-man" elements.