User blog comment:B214/Volume 23 discussion blog/@comment-191.214.24.46-20170529011323/@comment-29963364-20170529054322

In Real Life Myth, Midgardsormr will awaken at the beginning of Ragnarok, and take part in the final battle between the gods. During the battle, due to his great size, his movements will create massive floods, tidal-waves, and earth-quakes throughout the world. This, combined with the clashing of several gods, giants & monsters will leave the world in ash & rubble. But the world will eventually be reborn with new humans being guided by one god.

In DxD, he's just asleep in some deep, remote location in the world, and he's actually surfaced multiple times in the past. One time, he moved to the surface while still sleeping, and was taken care of by the local Dwarves and Dark-Elves.