Monster waves should be added to the list of hazards faced by ship captains as they plot a course through the waters of the Arctic Ocean, according to a new study that reports observations of house-sized swells in seas that until recently were covered in ice year-round.

“Waves always pose a risk to working at sea,” study author Jim Thomson, an oceanographer at the University of Washington in Seattle, said via email to NBC News from off the coast of northern Alaska. “The unique thing about the Arctic is that it is changing so rapidly that we cannot apply past measurements to understand future risk.”