X-Men: Omega Point
In 1936, Carl Jung wrote an essay about the psychological force behind the Nazi Party in his native Germany as an insurgence of the Wotan archetype in the collective unconsciousness. German nationalists revived imagery of ancient Germanic gods like Odin, Thor, and Loki. This collective unconscious might have been considered metaphorical at best when Jung wrote, but the existence of telepaths like Charles Xavier, Jean Grey, and Emma Frost leant a certain degree of reality to these archetypes. Their recent invocation during the war left them closer to the surface than many other ancient gods and archetypes, and so they were able to seize upon images of polytheistic divinity and emerge into reality where other pantheons did not.
In 1969, these archetypes began to erupt into the physical world, birth through the minds of latent telepaths and even ordinary Homo sapiens with a certain sensitivity or connection to these archetypes, as in the case of Loki Laufeysson. In nearly every case, the birth of a god had the unfortunate side effect of completely obliterating the human mind that existed before. The sole exception was Thor Odinsson, who shares his body with Dr. Donald Blake.
Thor was reborn when Dr. Blake was on vacation hiking in the Scandes on the border between Norway and Sweden. There, in a cave, he discovered Mjolnir. Upon touching the hammer, Thor was reborn in his mind. Sensing the return of the gods, Thor carved off the top of a mountain, lifted it into the air, and turned it upside-down, and proceeded to carve out a city &mash; Asgard — atop it.
Many ancient gods have re-emerged in the days since, including Heimdal, Sif, Tyr, and Frigga. Odin has returned as well to claim kingship over the city. The place of Asgard in international affairs has yet to be settled.