In 942.M41, Commissar Ciaphas Cain, on a mission in the Ice World, Nusquam , discovered hibernating Tyranids buried deep in the permafrost; the swarm was apparently carried to the planet by a Hive Ship that had crashed on the planet, prior to any human colonization.
In 997.M41, Hive Fleet Leviathan appeared from "below" the plane of the galaxy (on the Z axis) and attacked from two points, cutting off large portions of the galaxy from reinforcements.