Bleeding Cool is reporting that a portion of the film will be set underground and that in addition to studio sets the production will also use actual underground locations.
Some long tunnels in London will be used for the filming of scenes that, as yet, we haven’t been able to confirm as taking place on Earth, on Asgard or elsewhere. A long tunnel is a long tunnel, though, and when it’s dressed it could be anywhere. But a good long shot, down a real, long tunnel can’t be easily faked on a soundstage.
This bit of information also jibes with some of the rumors circulating that the sequel will feature Norse mythology’s and the comics’ version of elves, who dwell in caves in the realm of Svartalfheim.
Previously, Mads Mikkelsen had been offered a role in the film but passed in favor of taking the lead in the NBC pilot Hannibal. Bleeding Cool is speculating that perhaps the role Mikkelsen had been offered was that of the ruler of the Dark Evils, Malekith. Malekith has allied himself a number of times with Loki, so it should be interesting to see a variation of that dynamic play out in the film. There is no word yet as to who may be replacing Mikkelsen.
It should be noted that in the comics, Malekith at one point possessed the mystical artifact known as the Casket of Ancient Winters, which we saw in the first Thor film being used first by the frost giants when they attempted to invade Earth in 965 AD and again later being used by Loki to incapacitate Balder in his attempted takeover of Asgard. I wouldn’t be surprised if we see it make a return appearance.