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.They'd tried. Two expeditions twelve years apart had vanished without trace without explanation other than the dusty notebook Andrew had unearthed today from the rotted shreds of a skeleton's clothing.