Natalie Portman and Bear Grylls Filter Water Through His Underwear in New 'Running Wild' Clip

There's "going commando" and then, there is Bear Grylls's version of "going commando."

In the upcoming premiere episode of National Geographic's Running Wild with Bear Grylls: The Challenge, Natalie Portman gets to traverse the slot canyons of Southern Utah with the intrepid explorer. But little did the Oscar winner know that her guide would teach her a new meaning to the phrase "going commando."

As they search for drinking water, the two come across a shallow pool of sludgy water. Grylls explains that they can consume it, but first, they will need to filter and boil it.

"This water is really full of loads of dead flies and dirt and everything," Grylls explains.

After collecting the water in two large flasks, Grylls ponders what they could use as a filter.

"We could use a sock, but then again, you kinda need your sock," he says, and then offers up another suggestion — his underpants.

"We're gonna filter sludge water through underpants?" Portman, 41, asks incredulously.

Grylls then turns away from the actress and begins to cut the waistband of his Calvin Kleins, expertly removing what remains of his underwear without taking off his pants.

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"I mean it's not going to be perfectly clean," he says, to which Portman replies, "That's an understatement."

As the outdoorsman pours the water over the underpants filter, Portman makes an assessment.

"This is … gross," she says.

But Grylls is happy with the results.

"Look at that!" he exclaims. "Perfect."

Portman is the first guest to appear in the new iteration of the show, in which Grylls teaches celebrities "essential survival skills that they will have to master and then prove they can use in a high-stress situation," according to a summary.

This season's guests and locales include Simu Lui in the Canadian Rockies, Ashton Kutcher in the jungles of Costa Rica, Florence Pugh in the volcanic rainforests of Costa Rica, Anthony Anderson in the Sierra Nevada Mountains and Rob Riggle in the Great Basin Desert.

Running Wild with Bear Grylls: The Challengepremieres Monday at 9 p.m. ET on National Geographic.