This nuclear-powered project in the Arctic was featured a couple of times in National Geographic in the early 1960's, and those of you that have the magazine on CD-ROM should be able to access more information about this fascinating study in polar living.
The U.S. Army Polar Research and Development Center built an entire city under the ice, in Greenland. The lower cover is one from 1963 with a stamp honoring one of Missouri's "hometown boys" franking the envelope, General John "Black Jack" Pershing, franked with a plate block single no less, and postmarked with a machine cancel from Thule Air Force Base, Greenland-APO 23. They spruced their cachet up a bit from the one used in 1961!