The beaching of 70 dead gray whales this year along the West Coast, from California to Alaska, has been called an “unusual mortality event” by the National Oceanic and Atmospheric Administration Fisheries. The agency said researchers are still working to determine whether the deaths were caused by disease, human activities, or environmental factors. At least 37 dead gray whales were found in California, 25 in Washington state, five in Alaska, and three in Oregon, according to the NOAA. Another five dead whales were found in British Columbia, in Canada. A gray whale die-off in 1999 and 2000 was also declared an “unusual mortality event” and was believed to be caused by an ocean-warming El Nino event, Reuters reports.