Reminiscent of the Cold War, this page discuses some sort of weapon that was tested in these caves.  The “squares of honeycomb-textured metal,” (158) which are “from a casing [Vévoda] use[s] at the factory” (159).  Embedded in the wall, the casing is most likely from some sort of bomb, which exploded.

Published in 1949, S.O.T. had to have been written during the entire Cold War, with plenty of effects from the time period.  During the Cold War, in the early 1950s, the United States tested their nuclear weapons underground, similarly to how they were tested here.  Originally they were going to be tested at Amchitka, an island off the coast of Alaska, however that location later was moved to Nevada.  When researching Amchitka, there do not seem to be a ton of similarities with the island from S.O.T. and it.  While they are both mountainous and full of vegetation, Amchitka is most likely nothing more than a slight inspiration for this island in S.O.T. However, the reference to this island, and the Cold War in general, most likely means that Straka was in the sphere of influence of the Cold War. Implying Summersby as a likely candidate.

As far as F.X.C.’s note about an “experiment gone awry” goes, there seems to be a fair amount of biological testing done by the United States in the mid to late 1940s.  While what is being described in the story is almost certainly a bomb, Bouchard could have stumbled upon some of the biological warfare mentioned above.

All together, this entire event sounds a lot like American terror of the Soviets during the Cold War.  Perhaps this will run through the novel later on, and should be explored further.