Dead and dying bottlenose dolphins are washing up along the Mid-Atlantic coast at an alarming rate. Since June, more than 120 dolphins have appeared on beaches from New York to Virginia—seven times the typical number. Early this month, the National Marine Fisheries Service declared the strandings “an unusual mortality event” requiring immediate attention. “All indications show there’s something serious going on,” Trevor Spradlin, an NMFS marine biologist, tells NationalGeographic.com. Preliminary tests on some of the dolphins have shown evidence of morbillivirus, an infection responsible for one of the largest dolphin die-offs in history. Between 1987 and 1988, more than 700 dolphins with morbillivirus washed up dead along the East Coast, and many more likely perished offshore. It could take researchers several months to determine what’s ravaging the dolphin population this time.