chunk: The U-33 is now low on fuel, with its provisions poisoned by the saboteur Benson. 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. A freshwater current guides the sub to a stream issuing from a subterranean passage, which is entered on the hope of replenishing the water supply. 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. As the sub travels north along the island’s waterways the climate moderates and wildlife undergoes an apparent evolutionary progression.