The Immortals of Mercury

An explorer caught by the aboriginals of Mercury, who is tied to the back of a salamander-like monster that carries him away into the sun-ward deserts of the planet. Almost swooning with the insupportable heat, he sees the lifting of an artificial lid in the desert floor, and is rescued and carried into subterranean regions by strange beings who have achieved immortality by wearing clothes of a material that excludes the destructive cosmic rays.