This volume assembles four classic stories from the author of "Way Station" and "Project Pope." From time travel to hunting alien pests, from invaders from the fourth dimension to impossible sexless alien biology, Clifford D. Simak plumbs the depths of science and the heights of adventure in these classic tales Included are: "The World That Couldn't Be," "The Street That Wasn't There," "Hellhounds of the Cosmos," and "Project Mastodon."