Writers have long used Mars as a locale for encounters with alien beings. It is one of the planets nearest earth, and it has dimensions and geography that resemble earth to a great degree. In "The Hole Man," Niven presents the time-honored expedition to Mars, going there not to find alien beings, but to survey and record the planet's surface. Careful about his science, Niven makes sure that a scientific instrument intended for researching the planet's geology is plausibly able to detect the abnormality that turns out to be an artificial structure: "The base was an inverted pie plate two stories high, and windowless. The air inside was breathable, like Earth's air three miles up, but with a bit more oxygen. Mars' air is far thinner, and poisonous. Clearly they were not of Mars." In the space of a short story, Niven conjures up a fascinating place that...