With the movie The Avengers making over $1 billion world wide, I started thinking about Norse Mythology and the nine realms or worlds associated with it:
Ásgarðr, world of the Æsir.
Vanaheimr, world of the Vanir.
Álfheimr, world of the Álfar (Elves).
Miðgarðr, world of humans.
Jötunheimr, world of the Jötnar (Giants).
Niðavellir, world of the Dvergar (Dwarfs).
Múspell, world of fire and the Fire Jötnar.
Niflhel, world of ice and mist, into which the wicked dead are cast.
Hel, world of the inglorious dead, located within Niflhel/Niflheimr and ruled over by the giantess Hel.
The worlds of the elves, giants and dwarfs do sound like they could be extraterrestrials from some of the encounters out there. I think this has all been explored before, but I do wonder if any of these places could also be real locations on our own planet. The world of ice known as Niflhel could be Greenland (or memories of the last Ice Age); the land of fire could be Hawaii. I do know the Norse were very seaworthy and could of made it to the Pacific Ocean.
I am going to say the worlds of the beings listed are extraterrestrial, and the others can be found on Earth.