When Asgard was relocated to Midgard, the World Tree Yggdrassil moved with the home of the Asgardians. At the new site, an expedition was ordered by Odin where both Thor and the Lady Sif journeyed to the depths of the World Tree in order to reach its root. After fighting off the World Tree's insectile defenders, they managed to reach the bottom of Yggdrassil where they encountered the Worldheart which they took to the surface. Thor, however, felt a great danger would come to pass by the recovery of the Worldheart and above the planet the Silver Surfer sensed the great power of the seed of Yggdrassil that could feed his master, Galactus.
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