Fleeing a carnivorous race of alien monsters, the entire surviving human population from five hundred years in the future escapes into the present in this thrilling science fiction adventure from one of the Golden Age greats Our human descendants from five centuries in the future are coming to visit--all one billion of them--arriving via tunnels through time. Even though the present is merely a stopover and their ultimate destination is the age of the dinosaurs, their arrival has caused a worldwide uproar. Some folks want them gone and some want to go with them, as governments and powerful corporations alike scheme to get their hands on remarkable, potentially profitable time travel technology. There is a dark and terrifying reason, however, for the visitors' abrupt arrival. Our frightened descendants are seeking sanctuary from carnivorous aliens who have descended upon the future Earth, a threat that could mean the rapid destruction of the entire human race. And the end could come sooner than anyone imagined--for some of the intelligent, rapidly breeding extraterrestrial monsters who have been devouring our children's children may well have followed their prey back to the now.   A speculative fiction master who stands alongside Asimov, Clarke, and Heinlein in the pantheon of Golden Age science fiction gods, multiple Hugo and Nebula Award-winner Clifford D. Simak delivers an alien invasion tale that is at once wildly imaginative, seriously thought-provoking, and just plain fun.