The Existence

U-2 and A-12

"The government formally acknowledged the existence of Area 51 for the first time in 2013 when the CIA declassified documents about the development of the U-2 and A-12. Area 51 is still an active base—but the purpose it has served since the 1970s is a top-secret mystery. It will be a few more decades, at least, until current work is declassified and available to the public" (Combs). 

 UFO

Area 51 "is only one of many military bases, national laboratories, and government scientific research centers across the country that deal with classified — even Top Secret — information, and where workers and visitors need security clearances" (Lea).

"There's no reason to think that anything UFO-related is going on," Radford said (Lea).

Oxcart

"Area 51 had to be expanded because the OXCART, one of the planes tested there, took almost 200 miles to turn around because of its high speed" (Britannica).