An exploration into the more obscure and interesting corners of aviation
I'm betting either they are above ground tie-down areas or the hangar structures have been removed. Hangars that rise out of the ground and then go back down, like aircraft carrier elevators, would be pretty cool though. I once heard of an engineering project looking into underground airbases (runways and hangars) planned in Israel. Aircraft would fly in and out of an opening. Don't know if anything ever became of it though.
I have since done a bit of research, and discovered that the airport may have been built by the founder of the Land's End company, based in nearby Reedsburg, WI. I suspect it might still be owned and used by the owners of the company, and/or by the company itself.
Yes, you are correct. Those are underground hangars and yes it's owned (or at least was built by) the founder of Lands End. His son tried to start an air cargo company back in the late 90's with a Caravan based in MSN, that might be the one you saw the flight plan for. There is a private instrument approach serving the airport too.