In the 1980s, the USAF brought a fighter pilot into a remote Nellis AFB compound and asked what he thought about a plane that would be invisible to radar.
His response was simple: “Impossible. It can’t be done”.
Their response: “It’s called Nighthawk, it’s sitting 200ft away, and we’ve been flying it for years.”

That came out of a 1970s Area 51 project called Have Blue. The world would not find out about this aircraft until 1988 — 11 years after the demonstrator first flew and 5 years after the Stealth Fighter became operational during the tensions of the late Cold War.
That is the type of futuristic tech that is being developed and protected in utter secrecy at Area 51.
It has a whole lot of these:

And these:

It was also the site used to design and develop this:

And this:

And this:

In case you haven’t noticed, enemy spies having access to any of these, or the information needed to make them, would be very very bad. The last thing we want is the Russians making their own B-2 bombers.
Groom Lake is where the SR-71 Blackbird spy plane was tested.

It’s where the F-117 Nighthawk stealth fighter was tested,

It’s where a lot of stuff critical to national security that the agencies don’t want potential adversaries to know about is tested.
And if you go poking around without permission, they can, will, and should detain, arrest, or shoot you, depending on circumstances.
