The funny thing is that Area 51 (Groom Lake) wasn't particularly secret or guarded by the CIA. When they started construction there in 1955, the area was known as Watertown, and they sent out press releases. "Construction at the Nevada Test Site installation a few miles north of Yucca Flat which was announced last spring is continuing. Data secured to date has indicated a need for limited additional facilities and modifications of the existing installation. The additional work which will not be completed until sometime in 1956 is being done by the Reynolds Electrical and Engineering Company, Incorporated, under the direction of the Atomic Energy Commission's Las Vegas branch office."
There was a no fly zone for the area; a nearby testing site for the AEC was already protected, and so the area was enlarged to include Groom Lake. The development of the U-2 was also mentioned, but it was called a high altitude atmospheric research plane. Up to this time, anyone had access up to the site and could see what was happening. It wasn't until June 1958 that 38,400 acres of land were removed from public access, mostly to the south. This chunk of land was designated Area 51. There was still an open area to the north with unpaved roads where ranchers, miners and curiosity seekers could get close and take a look.
Until that point, the base was considered temporary. After the decision in Jan. 1960 to use the area for the work on the Lockheed A-12, the place became permanent and began expansion. Buildings for storage, administration, testing, a fuel farm, gym, theatre, golf course and softball field were put in. An unclassified newletter (N.T.S. Bulletin) often published local Area 51 events and headlines like "Area 51 Wins Slow-Pitch Tournament" were common and many of the players were listed by name. Still not really secretive, although unlike the U-2, the A-12 project was never publicly mentioned.
Over the years, the Area 51 location and mention were in several unclassified maps. An unclassified NTS road map, produced by Holmes & Narver, Inc., was updated in January 1961 to include mileage between various points. One notation read: "10.3 MILES TO AREA 51" and was followed by the words "GROOM LAKE." A map showing the area as visited by President Kennedy in 1962 pointed out Groom Lake, Watertown and the Area 51 boundary. Maps by the AEC showing the Nuclear Test Site in 1977 still showed Area 51 and Groom Lake clearly labeled. A 1978 Bureau Of Land Management map showed lots of detail of the area including the roads and landing strip, all unclassified.
It wasn't until 1979 when the USAF took control of the area from the CIA that active measures began to be taken to clamp down on secrecy, excluding hunters, hikers and miners from the area and in 1981 took over the northern area as well, having another 89,600 acres withdrawn from public use. And that's when the real era of secrecy had begun.