I'm William Brangham and this is Horizons.

Unexplained sightings have appeared in our skies for years.

Oh, yeah, they do you.

And now, the U.S. government has started releasing declassified files about what we used to call UFOs.

The aliens are real.

Well, I don't know if they're real or not.

What explains these phenomena and what do we know about life beyond Earth?

Coming up next.

Welcome to Horizons. We are talking today about one of the most profound questions facing humanity,

one that has intrigued generations and sparked widespread conspiracies, and that is, has Earth been visited by a non-human intelligence from somewhere else?

There are many serious people in the government, at academic institutions,

and in the U.S. military who believe that we have.

Part of that is driven by things like this.

There's a whole fleet of a look on the essay.

My gosh. A series of encounters, often filmed by the U.S. military, of what appear to be objects flying

in the sky at remarkable velocities with no obvious form of propulsion.

It's rotating.

These sightings are not rare or isolated.

They are routine.

Military air crew and commercial pilots, trained observers whose lives depend on accurate

identification, are frequently witnessing these phenomena.

While the U.S. military has confirmed many mysterious sightings, other former officials acknowledge not being totally

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