What was found
AT APPROXIMATELY 16,000 FT DURING CLIMB OUT OF PDX THE AIRCRAFT EXPERIENCED A RAPID DECOMPRESSION. IT WAS DISCOVERED THAT THE L/H MID EXIT DOOR PLUG STRUCTURE BLEW OUT AND DEPARTED THE AIRCRAFT. THE FLIGHT CREW INITIATED AN EMERGENCY DESCENT AND DECLARED AN EMERGENCY. THE FLIGHT LANDED SAFELY IN PDX TWENTY MINUTES AFTER DEPARTURE. GROUNDED - YES. THE AIRCRAFT IS CURRENTLY GROUNDED. ROOT CAUSE INVESTIGATION IS ONGOING.