In 1965, Annie Jacobsen explains (in Area 51), Colonel Slater took over command of Groom Lake:
Slater was the ideal commander. He was astute, practical, and an excellent listener, which put him in direct contrast to the more elitist Colonel Holbury, who’d held the position before. What the pilots appreciated most about Slater was that he was funny. Not sarcastic funny, but the kind of funny that reminded pilots not to take their jobs so seriously all the time. One of the first things Colonel Slater did after taking command of the base was to hang a sign over the House-Six bar that listed Slip Slater’s Basic Rules of Flying at Groom Lake. There were only three rules.
Try to stay in the middle of the air.
Do not go near the edges of it.
The edges of the air can be recognized by the appearance of ground, buildings, sea, trees and interstellar space. It is much more difficult to fly there.