What is Gunbuster?

A 1989 OVA about girls with big boobs who pilot a 200 meter tall super robot to save the human race from evil Space Monsters (Uchu Kaiju) that want to "purify" the universe by wiping out humanity. The series makes use of realistic physics when it comes to the use of relativistic speeds, and features what is probably the second most destructive giant robot in an anime (after the Ideon).