Poseidon's Wake

Two hundred years after the fall of Mechanism, human society has achieved a kind of stability. There are colonies beneath the oceans, throughout the solar system, and beyond: on the worlds of extrasolar systems. Vast hemirelativistic ships connect these colonies, travelling at half the speed of light. Or rather they would, if the ominous presence of the alien Watchkeepers had not led to an enforced moratorium on interstellar travel. But when a seemingly impossible radio signal reaches the colony Crucible, everything changes: Send Ndege Its origin is unpopulated, unexplored space. No one could be there at least, not if they travelled using human technology so who could have sent it? How did they get there? And what use do they have for the disgraced scientist Ndege Akinya? Finding the answers will require one of the greatest expeditions humankind has ever launched, a journey further than ever attempted before, conducted under the implacable scrutiny of the Watchkeepers. But as a mission is prepared on Crucible, it turns out they werent the only ones to see the message or its potential...