It's to the best of our knowledge.

I'm Anne Strangeamps.

Today, our national parks.

Oh my goodness, just across the little river about 150 yards away from me is a black wolf.

Moving quickly right along the edge of the willows.

And that is why millions of people every year

visit America's national parks

for the chance to see wildlife up close,

no cages, no fences, just the raw encounter,

where you never know what might happen.

Oh, dark, enormous moose,

swaggering, casually and confidently right in the direction of that

grizzly bear. Oh my God. That moose is running straight toward me. It's gotten wind of the grizzly bear.

And holy macro, life is exciting. Where was I?

That's Richard Nelson, recording on location in Denali National Park.

He hosts a radio show from the Alaskan Wilderness,

where his guests are wolves, moose, and grizzlies fighting over wild salmon.

But the national parks aren't just something to enjoy.

People fought to establish them, and they're at the center of renewed political debate today.

So to put some of those battles in context, Steve Paulson talked with historian William Cronin.

He says the idea of the national parks is a key part of our cultural heritage.

I think you don't understand the United States if you don't understand first that Americans chose to set aside these tracts of land and then ask the question, why did they set aside the particular tracts of land that they did?

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