Watching Elon Musk watch the Falcon Heavy liftoff Tuesday, you might think he didn't expect success.

"Holy flying f---, that thing took off," Musk says seconds after the rocket launched.

That's according to exclusive video from National Geographic of Musk in the control room at NASA's Kennedy Space Center in Cape Canaveral, Florida.

National Geographic released a clip of the video.

Exclusive behind-the-scenes-footage follows Elon Musk in the moments before the Falcon Heavy launch

The footage from Tuesday's launch was shot for the second season of National Geographic's show, "Mars."

The video shows Musk running outside and gawking at the sky seconds after Falcon Heavy took off. "Look at that! That's unreal!" he says. It was a milestone because the successful launch makes the rocket the largest in operation today.

Musk also shared two images of himself from the clip on Instagram.