Jason Momoa's menacing scowl in "Aquaman" is finally explained.

It's probably pain from the hip-hugging rig and harness required to shoot scenes replicating underwater movement. The results look stunning in the DC Comics deep-sea extravaganza, which dominated at the box office this weekend.

But director James Wan acknowledges that the hours of shooting were painful on Momoa's nether region.

"(Jason) wasn’t the biggest fan of the swimming rig we had to put him in. It’s not the most comfortable rig for actors," says Wan. "It puts a lot of pressure around the crotch. So for someone like Momoa, who is more than 200 pounds, it’s not the most pleasant experience."

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Wan's technical crew had to devise a system that would allow the characters to "float" in their underwater world.

They developed a tuning fork-type rig in which the actors were suspended from hip harnesses. Shot against blue screen (with the water added later through CGI), the rig allowed for realistic movement and dialogue.

"If they are floating around and talking to each other, they have to be harnessed into these rigs," says Wan. "It was a necessity for the film."

Momoa, who has two children with wife Lisa Bonet, used to joke about with Wan about the long-term effects of the snug harness.

"Jason would to say to me, 'James, I don’t think I’ll be having more kids anytime soon,' " says Wan.

Momoa, Patrick Wilson (as King Orm) and Dolph Lundgren (King Nereus) even came up with a system to help ease the pain during long shoots.

"One of the things they would do to keep the pressure off their (midsection) is they would lean forward, and sort of spin themselves upside down," says Wan. "That allowed the pressure to come off."

Bright side: The scenes are striking, and the method certainly beat shooting in frigid water, as Momoa did for "Justice League" in England.

"That was an eye-opener. England is really cold and we were wet often," says Momoa. "But you’re playing Aquaman. What am I going to do, cry about being wet? Do your damn job, actor boy."