AMNA NAWAZ: As the country celebrates its 250th anniversary, a growing threat is putting some of America's most historic places at risk.

Rising seas and coastal erosion are reshaping the landscape where our history unfolded.

Special correspondent Ben Tracy with Climate Central takes us to Jamestown, Virginia, where archaeologists are racing to uncover America's past before it's washed away.

It's part of our ongoing series Tipping Point.

WOMAN: Well, and we know that it's shallow.

BEN TRACY: If you spend your days digging through the past... SEAN ROMO, Jamestown Rediscovery: I think it might be a gate.

BEN TRACY: ... Sean Romo says this is the place to do it.

SEAN ROMO: You can't put a shovel in the ground without finding something.

BEN TRACY: Romo is director of archaeology for Jamestown Rediscovery at the site of America's first permanent English settlement founded in 1607.

What took place here and why does this matter?

SEAN ROMO: Well, for American history, it's harder to have more of an impact than Jamestown.

This is the first place that, when English colonists come over here, they end up staying.

And without Jamestown, there is no modern United States.

BEN TRACY: This is where America's first representative assembly met, where Pocahontas married John Rolfe, and where the first enslaved Africans were forced onto these shores.

SEAN ROMO: And they are kept enslaved by the English colonists.

So in one place, here at Jamestown, we have our ideals of American democracy and our biggest mistake in slavery.

So this is a place where many different cultures interacted.

Many different cultures have a claim on the site, and the history is pretty complicated.

BEN TRACY: The ruins of the original fort were discovered in 1994.

SEAN ROMO: We can just open this up a little bit.

BEN TRACY: And archaeologists have unearthed five million artifacts that tell the stories of the people who lived here.

SEAN ROMO: This is a chain that was found at the bottom of one of Jamestown's early wells.

And you can actually hold it if you like.

BEN TRACY: So this is from how long ago?

SEAN ROMO: This is from the beginning of the 17th century.

BEN TRACY: Now they're using ground-penetrating radar to map out what still lies buried.

SEAN ROMO: Now, if it shows up, that will be really cool.

BEN TRACY: Because this has become triage, a race to save as much history as possible before it's lost.

When I think of archaeology, I think of a very slow, painstaking process.

You, however, don't have time on your side here.

SEAN ROMO: No.

We always have to be meticulous, but we do need to pick the pace up, because we are under severe threat from climate change.

And the real big one for us is flooding.

BEN TRACY: Jamestown is under siege from rising waters.

Sea levels here have risen 1.6 feet over the past century and could rise another three feet or more by 2075.

The James River is battering the shoreline on one side.

That swamp is really close.

SEAN ROMO: Yes, and it didn't used to be.

BEN TRACY: While an expanding swamp on the other regularly floods the excavation sites.

So what happens on a day where you get a lot of rain or it's high tide?

SEAN ROMO: On a day with a lot of rain or high tide, we're in the water.

BEN TRACY: And so when you talk about history being washed away, it's a literal thing.

SEAN ROMO: Oh, yes, that's not hyperbole.

It's actually happening right here.

BEN TRACY: And it's not just here.

Our burning of fossil fuels is rapidly warming the planet, causing ice sheets to melt and ocean water to expand as it heats up; 2.5 million Americans and many historic sites could be at risk of severe coastal flooding by 2050.

ROB YOUNG, Western Carolina University: If you want to argue about why or who's causing it, great, knock yourselves out, but sea level is hose rising.

We have been measuring it for more than 100 years.

BEN TRACY: Geologist Rob Young in his team at Western Carolina University are now assessing the risks to all 107 coastal national park sites for the National Park Service, including the Statue of Liberty, Fort Sumter, and Pearl Harbor.

ROB YOUNG: We have increased flooding of roads, parking lots, it impacts visitor access, all kinds of changes like that parks just haven't had to deal with before.

BEN TRACY: What are we most at risk of losing?

ROB YOUNG: The greatest vulnerability in the national park system without question are the barrier island parks in the Southeastern U.S.

BEN TRACY: The Atlantic continues to swallow homes along North Carolina's Outer Banks, which is why the historic Cape Hatteras Lighthouse was moved to quarter-mile inland back in 1999.

But it's one thing to move a lighthouse, another thing to move a Civil War fort.

ROB YOUNG: Yes, we're not going to move Fort Sumter.

So, no, it's not going to look like what it looks like now for your kids or for your grandkids.

Honestly, we can't protect them all.

And the trick for us as a society is to admit that and have a good national conversation about what we do about it and where our national priorities are.

But, unfortunately, I just don't see us having that conversation.

BEN TRACY: So, Sean Romo will continue to dig as long as he can, while Jamestown reinforces its defenses, hoping to buy more time.

Do you see a time where Jamestown Island is basically underwater?

SEAN ROMO: Sadly, yes.

If we do nothing right now, we're going to go from Jamestown Island to Jamestown Islands in the next 50 years.

So the time is now to act to protect this space and to make sure that future generations can still learn about and experience the place where American history was made.

BEN TRACY: For the "PBS News Hour," I'm Ben Tracy with Climate Central.