DEN = FAA abbreviation for Denver. However, the airport is universally referred to as DIA, "Denver International Airport." DIA shows on maps, busses, street signs, and on the news. But the bag tags there say DEN.
Denver - one of the worst set up airports that I have had the misfortune of flying into. I was trying to get a bus from the terminal to the rental car area (even the "on airport" companies are located several miles trom the main terminal.) I stood at the designated stop, but all of the buses were full and wouldn't stop. I finally walked most of the distance along the front of the terminal (unshaded and in the summer heat) until I got to the first terminal stop and managed to squeeze onto a bus.
No wonder that so many of the airlins servicing Denver have filed bankruptcy.
It is an airport that I avoid.