Greg Bear – The Way

The 21st century was on the brink of nuclear confrontation when the 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...and discovered marvels and mysteries to drive researchers mad.
For the Stone was from space--but perhaps not our space; it came from the future--but perhaps not our future; and within the hollowed asteroid was Thistledown. The remains of a vanished civilization. A human--English, Russian, and Chinese-speaking--civilization. Seven vast chambers containing forests, lakes, rivers, hanging cities...

The crew of the asteroid-starship Thistledown has thwarted an attack by the Jarts by severing their link to the Way, an endless corridor that spans universes. The asteroid settled into orbit around Earth and the tunnel snaked away, forming a contained universe of its own.
Forty years later, on Gaia, Rhita Vaskayza recklessly pursues her legacy, seeking an Earth once again threatened by forces from within and without. For physicist Konrad Korzenowski, murdered for creating The Way, and resurrected, is compelled by a faction determined to see it opened once more. And humankind will discover just how entirely they have underestimated their ancient adversaries.

Now the rulers of Axis City have discovered that a huge group of colonists has secretly entered one of the interdicted worlds along the Way. In some ways Lamarkia is very Earth-like--but its biology is extraordinary. A single genetic entity can take many forms, and span a continent. There are only a few of these 'ecos' on Lamarkia, and the effect of human interaction on them is unknown.
Olmy Ap Sennon has been sent to secretly assess the extent of the damage. But he will find far more than an intriguing alien biology--for on their new world the secret colonists have returned to the old ways of human history: war, famine, and ecological disaster. On this mission, Olmy will learn about the basics: love, responsibility, and even failure...