This recently came back to me from Svalbard. The Polish Academy of Science has been operating an Arctic research station on Spitsbergen island since 1957. The base is situated on the north side of the Hornsund fjord, about 136 km south of Longyearbyen.
This isn't the envelope I sent up there, but those are the stamps I used. The original cover must have been damaged somehow (wouldn't be the first time) so someone salvaged the postage and sent me this instead. No postmark and only the "41st Polar Expedition" stamp marked on the front but there were some extras inside, including a couple of pages with various hand-stamps on them.