RENEE MONTAGNE, host:

All this week, NPR's visiting some of America's national parks. Today, we head to Alaska's Glacier Bay, a park that offers great natural beauty and, as NPR's Martin Kaste reports, may also help unlock some mysteries in nature.

(Soundbite of seal)

MARTIN KASTE: In the tiny town of Gustavus, Alaska, it seems everybody is an expert on a very peculiar geophysical phenomenon.

Mr. MORGAN DEBOER: Isostatic rebound is the term for it.

KASTE: Morgan DeBoer has good reason to be up on the jargon. Isostatic rebound is an uplift in the Earth's crust, and around here, the crust is rising fast. So fast, it's doubled the size of the DeBoer family's waterfront property.

Mr. DEBOER: You know, land rebounding, coming up, springing up out of the water at an inch and a half a year is something you wouldn't think it's much, but when it's flat land to begin with, any change at all is really magnified. We gain probably an acre or so every year.

KASTE: But what is it that's making the land under Gustavus rise up like this? The answer is out here in Glacier Bay. This whole fjord, all 60 miles of it, used to be covered by a massive sheet of ice. The weight of that super glacier pressed down on the Earth's crust, sort of like a bowling ball on a foam mattress. Then the ice melted. It was a natural phenomenon, probably not connected to man-made global warming. But once the bowling ball of ice was gone, the mattress, the Earth's crust, sprang back into shape. And the Earth is still springing up, no where faster than here on Russell Island. Two hundred years ago, this spot was under thousands of feet of ice. Now the island is surrounded by open water, and it's home to oyster catchers. That's these noisy birds that poke their long orange bills into the crevices between the rocks that were left here by the glaciers. The island also attacks another species: scientists - scientists lugging car batteries.

Mr. MAX KAUFMAN (Research Technician, Geophysical Institute, University of Alaska Fairbanks): It's the price we pay for doing science in the wilderness.

(Soundbite of laughter)

KASTE: Max Kaufman is a research technician from the Geophysical Institute at the University of Alaska Fairbanks. He and a team of researchers from Japan are dragging these hundred-pound batteries up the beach to power something called a gravimeter. Dr. Tadahiro Sato helps to set it up.

Dr. TADAHIRO SATO (Researcher): Yes, I study in gravity.

KASTE: Your specialty, that's what you studied.

Dr. SATO: Yeah, but I do not like gravity, because gravity is complex.

KASTE: Dr. Sato is probably kidding about not liking gravity, but gravity is surprisingly complex, especially here. The uplift in the ground is moving Glacier Bay away from the center of the Earth, and that changes the local gravity. So Dr. Sato and his colleagues want to get a precise measurement of gravity on Russell Island. And they'll do it by dropping a weight. That's all the gravimeter really does. It drops a weight down a tube, through a laser that measures its speed over and over and over again, 10,000 times. The researchers have to keep this temperamental $400,000 machine running smoothly for 48 hours, even as nature tries mightily to distract them.

Dr. WENKE SUN (Researcher): I hear the sounds, something like people walking behind me.

KASTE: At one point, Dr. Wenke Sun abandons the gravimeter and hurries down to the beach, looking a little freaked out.

So you heard steps in the woods.

Dr. SUN: Yes. So Iā€¦

KASTE: Did they sound like big steps?

(Soundbite of laughter)

Dr. SUN: Yes, so I just came through here.

KASTE: This part of Glacier Bay is home to brown bears. That's the coastal version of grizzlies.

(Soundbite of laughter)

KASTE: Back on the boat they rented, the researchers get a bear safety speech from Bonnie Harris. She's a local, and she knows bears.

Ms. BONNIE HARRIS (Bear Expert): The main thing is you just don't run, talk, laugh loud. Stay in a group.

KASTE: And a little later, they do spot a bear. It's a big brown, 600 pounds at least, and it's lurking just a couple hundred yards from one of their research sites. It stares at them from across the water, almost daring them to come back on shore.

(Soundbite of whistling)

KASTE: But go ashore they must, with one last load of batteries. Knowing the bear is somewhere in the bushes, they get the job done quickly. Once they have the gravimeter safely stowed back on the boat, they take a look at their preliminary results. It seems gravity in Glacier Bay is weaker than it was last year, seven parts per billion weaker. It's a tiny change, on the order of a whale losing the weight of a couple of hairs. But these small numbers shed light on the mysterious forces working deep inside the Earth. And there is one thing that has now been proved beyond the shadow of a doubt: Visitors to Glacier Bay can always look forward to a little weight loss.

Martin Kaste, NPR News.

MONTAGNE: Go ahead and tour the Alaskan wild in an audio slide show at npr.org.

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