Site 9 of the 556th Strategic Missile Squadron, Plattsburgh. One of 12 Atlas F ICBM sites in NY and VT which ringed Plattsburgh Air Force Base during the Cold War. The silos were 174 feet deep and 54 feet in diameter.
Now used as town garage.
See also:
http://www.atlasmissilesilo.com/556thSMS.htm
http://atlasbases.homestead.com/
http://www.556sms.com/