Originally Posted by Avenger
According to the timeline in The Art of the Avengers, Odin left the Tesseract on Earth around 600 years before IM1. I have no idea why he did it, though. Seems kind of risky leaving something that powerful on Earth.
Maybe as a measure so we could defend ourselves should the Frost Giants invade again. It's also possible that it was lost to Earth, not placed there.