[9] The leitmotif of the story is a journey to the center of the Earth[10]: 120 in search of valuable raw materials[11]: 160 through the use of technology allowing penetrating or passing through matter.
[12] Harrison wrote that he based the story's outline on a classic Western plot and the idea of claim jumping, but repackaged it with a sci-fi setting.
[14]: 126 Likewise, in 1985 Jerry Pournelle noted that the story "had the kind of idea that immediately engages the SF addict".
[15]: 19 Brian Stableford listed it as one of "notable accounts of burrowing expeditions" into Earth that are relatively consistent with the knowledge of modern geology.
[16]: 138 A reviewer for Analog Science Fiction and Fact wrote in 1969 that it is "highly realistic yarn about prospecting and claim-jumping inside the Earth.