The gray, two-story home with white trim toppled and slid, crashing into the river below as rushing waters carried off a bobbing chunk of its roof. Next door, a condo building teetered on the edge of the bank, its foundation already having fallen away as erosion undercut it. The destruction came over the weekend as a glacial dam burst in Alaska's capital, swelling the levels of the Mendenhall River to an unprecedented degree. The bursting of such snow-and-ice dams is a phenomenon called a jökuhlaup, and while it's relatively little-known in the U.S., researchers say such glacial floods could threaten about 15 million people around the world.