My guess (not worth much) is that it is an underground military airfield under construction (and a major effort, like few others anywhere in the world...as most of it is underground... and the underground area is where the aircraft would be stored, maintained, fueled, and readied for take-off when ordered). As such, it would provide hardened passive defense for an otherwise vulnerable forward deployed airfield (near the DMZ) and makes it extremely difficult to determine the Air Order of Battle (AOB). Hence, it inhibits early warning of an iminent launch of aircraft and the number and type of aircraft in that launch. The presumed runway extends out both sides of the tunnel (note that it is straight...a necessity for airfields) for a total of 1500 meters or ~5000 feet(measured with ruler on Google Earth...pretty short for fighter aircraft that use nominal runway lengths of about 12000 feet... also provides dual egress (and ingress?...that would be scary). Perhaps if there was a launch assist mechanism inside the mountain on the runway (like on aircraft carriers) this could still be used for fighter/bombers. It would be interesting to know the prevailing winds in the area to see how well they line up. Oh, I also did some checking, using Google Earth to determine the date of the imagery from Digital Globe, this image was acquired on 12 November 2002 (wonder how it looks now?) Two more things, 1) the 12 November 2002, Digital Globe image was centered, smack-on, another airfield that measures ~7900 feet , and, 2) If you click on the Google Earth terrain layer and then view the tunnel "airfield" in perspective, it adds additional strength that this is indeed an airfield as the "fly-out zones" are relatively clear of impeding terrain.