The first sounds came at midnight in a plaintive scream from an unknown voice in the vastness of space. Within hours, the entire world heard the eerie throbbing. And in billions of earthbound minds the horror grew. Hours later, to the ears of a helpless world, the second message came...Earth's days were numbered!Murray Leinster's horrifying epic of science fiction is still a thrilling read, 50 years after its original publication!