This system is only hinted at in The Land That Time Forgot; presented as a mystery whose explication is gradually worked out over the course of the next two novels, it forms a thematic element serving to unite three otherwise rather loosely linked stories.

A large island ringed by cliffs is encountered, and identified as Caprona, a land mass first reported by the fictitious Italian explorer Caproni in 1721 whose location was subsequently lost.

The U-boat surfaces into a tropical river teeming with primitive creatures extinct elsewhere; attacked, it submerges again and travels upstream in search of a safe harbor.

It enters a thermal inland sea, essentially a huge crater lake, whose heat sustains Caprona's tropical climate.

Tyler rescues Lys from a group of Sto-lu ("hatchet men"), and later aids the escape of a woman of the Band-lu (spearmen) to the Kro-lu (bowmen).

Unable to find his way back to Fort Dinosaur, he retreats to the barrier cliffs ringing Caspak in a vain hope of attracting rescue from some passing ship.

Galaxy reviewer Floyd C. Gale, discussing a 1963 reprint, described the novel as "sheer, headlong adventure that is unusual even for an ERB thriller.

It featured people from the present interacting with World War II troops on a mysterious, prehistoric island much like the one Burroughs created.

In July 2016, publisher American Mythology Productions released a comic book sequel by writer Mike Wolfer and artist Giancarlo Caracuzzo.

Starring Michael Paré, it featured a Russian attack on an Australian Navy vessel in the Bering Sea, with a gravitational anomaly marooning the crews on a time-displaced Caspak.