Eon - Greg Bear

Above our planet hangs a hollow Stone, it came from out of nothingness and into Earth's orbit. Within it there are a vast number of chambers to explore, containing forests, lakes, and rivers.

Some chambers even contain deserted cities, and the remains of a lost civilisation. Scientists determined that the Stone was from space... but perhaps not our space; that it came from the future...

but perhaps not our future. While scientists and politicians bicker about how to use their newfound discoveries, the Stone harbours a secret that makes even Earth's survival pale into insignificance.