The Dark Intruder

Xanadu. Not the Xanadu of Coleridge's poem, but-to the half-forgotten space drifter who discovered the place thirty years ago-a reasonable facsimile. It was a cloistered nun of a city, hidden behind a wide skirt of the most impassable mountains on Mars. And the city was more impassable than the mountains. No human being had ever entered it-yet.