Hi All,

After doing a bit of research I agree with Highground that it's an underground airfield and hanger facility.

See here: http://www.globalsecurity.org/military/world/dprk/airforce.htm

See also this post for another similar facility: http://bbs.keyhole.com/ubb/showthreaded.php/Number/85408

The runways appear rather short but I guess a MIG or similar fighter aircraft can get airborne in under 4000ft. It's the landing that worries me!
Heading straight for a mountain - how scary is that?