Bifröst

The rainbow called Bifröst is the connection between Asgard and Midgard. Generally, it can only be used by gods; it's usually closed to humans and giants. Furthermore, men are unable to find the place where Bifröst meets Midgard, because it vanishes when they get close. Some of them try again and again to set foot on it, but none have been successful so far. Actually, the same goes for giants but their attempts seem to be more fruitful. Therefore, Heimdall keeps watch over Bifröst, for he lives right next to it on the edge of Asgard in his hall named Himinbjörg. Nevertheless, the giants will find the rainbow at the beginning of Ragnarök, and the colorful bridge will shatter under their march, marking the prelude to the world’s downfall.