In cutting through ice, the muskrats are adept if they work from below, and they may work well at this in water. They may gnaw upward through the ice in the middle of a marsh, and then heap up the vegetation for a new lodge around the new hole. I happened to be walking across the care ice of a marsh center when a muskrat cut through ahead of me. It pulled its wet body out, and there it sat amid ice splinters beside a hole leading down through a foot or so of ice. A muskrat may similarly gnaw from below the ice over the plunge hole of a solidly frozen, abandoned lodge and rehabilitate the lodge in a few hours.
From OF MEN AND MARSHES by Paul L. Errington