In this novel, Bear offers what "may be the best constructed hard SF epic yet" (The Washington Post).
The 21st century was on the brink of nuclear confrontation when a 300-kilometer-long stone flashed out of nothingness and into Earth's orbit. NASA, NATO, and the UN sent explorers to the asteroid's surface. They discovered enough marvels and mysteries to drive all researchers mad.
The strange Stone was from space—but perhaps not our space; it came from the future—but perhaps not our future. Within the hollowed asteroid was Thistledown, the telling remains of a vanished civilization. Indeed, a human civilization—English, Russian, and Chinese-speaking—with seven vast chambers containing forests, lakes, rivers, and hanging cities.
Thistledown also contained museums describing the Death; the catastrophic war that was about to occur; the horror and the long winter that would follow. But while scientists and politicians bickered about how to use the information to stop the approaching Death, the Stone yielded a secret that made even Earth's survival pale into insignificance.