Until Thursday, the U.S. government had never publicly acknowledged the existence of Area 51—a remote spot in the Nevada desert that has long been associated with conspiracy theories, alien sightings, and talk of tiny spaceships.(See exclusive pictures from Area 51.)

But a newly declassified CIA document—which was written in 1992—shows a map labeled with the name of the remote land parcel, 75 miles from Las Vegas, used to test and develop top-secret U.S. military projects, including the U-2 spy plane. (Related: "How Area 51 Held Secret Craft.")

To learn more about Area 51 and the new information released by the government, we turned to Annie Jacobsen, an investigative journalist and the author of Area 51: An Uncensored History of America's Top Secret Military Base.

What was in this massive 400-page document that was declassified on Thursday?

Very little of it is actually new information. Only one page out of the 400 pages is new information. The rest of the report has been declassified for years. And I know this because I used large parts of it to write about the different surveillance programs that existed at Area 51.

So what's actually new?

What's new is that the National Security Archive at George Washington University filed a Freedom of Information Act request on a page of the document and it was declassified.

It is a map that says Area 51 on it. That really is news—what it means is that a jealously guarded secret—the existence of an area called Area 51, which the government refused to acknowledge the existence of—actually exists.

How was Area 51 referred to in other documents?

Whenever it was described, the words "Area 51" were covered in black ink. No one in the government would talk about what everyone else already knew.

What's interesting is that these incredible spy programs happened out there. The CIA has been declassifying documents about Area 51 since the 1990s, but they have always been told or directed not to reveal the name.

Why do you think that is?

It is still a mystery. I maintain that the Atomic Energy Commission was doing programs out there that required a much higher level of security clearance and those programs had a much higher degree of classification. (Learn more about Area 51 on the National Geographic Channel.)

What's in the rest of the document—the part that was already declassified?

It discusses various programs that existed out at Area 51. One of the first CIA drones ever created was flown out of Area 51 in the 1950s. It looked like a giant eagle and was meant to spy on Soviet hydrofoils that were being hidden in the Caspian Sea.

Why did people think that Area 51 housed alien spaceships?

A lot of it was because of the U-2 planes program that started out of Area 51 in 1955. The U-2 planes were used extensively by the U.S. during the Cold War for reconnaissance missions. They flew at an altitude of 60,000 feet (18 meters)—considerably higher than any other plane in existence at the time.

As soon as test flights began, people wrote letters to their congressmen and to their local mayors saying, "I saw a UFO." But what they saw was the U-2 spy plane.

Why did they mistake the U-2 plane for a UFO?

Two reasons. One, it had an almost vertical takeoff. Anyone who happened to see it gaining altitude would have thought it was very unusual.

The second reason is that it flew so high. At the time, no one believed that airlines could fly that high. Commercial flights were flying 20,000 feet (6 kilometers) in the air. If you saw [a U-2 plane], you were going to think it was a UFO.

Why are we so fascinated with Area 51?