Asgard's Secret: The Asgard Trilogy, Book One

They call it Asgard, the Home of the Gods. Beneath its artificial shell are at least three vast cave-systems, each one the size of an Earth-like world; and beneath those, possibly many more. No one knows how many layers there might be, and no one knows what secrets might be buried down at the "center"--if there is a center. At some time in the distant past, Asgard had suffered a terrible catastrop...