SUMMER EVENINGS IN SEATTLE are ideal for kicking back in the nosebleed seats at T-Mobile Park, hot dog in hand, rooting for the Mariners as the sun sets over the downtown Seattle skyline.

Games on either end of the season, however, are a different story altogether. Major League Baseball’s regular season runs from March to September, with 81 home games a year. Go to a game in April and you may well shiver like it’s November. But at least you’ll be dry.

T-Mobile Park’s retractable roof arches like a big umbrella over the stadium, the only retractable roof in the MLB that’s open on the ends. The home ballparks for the Houston Astros, the Arizona Diamondbacks and the Milwaukee Brewers also have retractable roofs, but theirs are fully enclosed, allowing for climate control inside. In temperate Seattle, that isn’t an issue.

T-Mobile Park’s roof spans 300 feet across and covers nearly 9 acres. It’s essentially an entire city block that moves. Peregrine falcons and red-tailed hawks have been known to nest in the roof structure where the steel comes down in a cradle. After Mariners’ wins, the roof trusses illuminate with a giant “W” like a bat signal.

“Outdoor baseball would be impossible without it in Seattle,” says Adam Gresch, senior manager of communications for the Mariners. “It’s the secret ingredient that makes all of the magic happen at T-Mobile Park.”

We’re walking along one of the two concrete runways that bookend the stadium, along which the machines that move the roof slide. There’s a north runway and a south runway; these are the only places where the clear span structure touches down.

From up here, the runway functions like a wide sidewalk scattered with bits of detritus (blackened fireworks debris, a desiccated bird wing) and offering dazzling views of the ballpark’s playing field and downtown. The runway is off-limits to the public, although an occasional TikToker has slipped in chasing dreams of virality.

Every few feet, circular holes perforate the concrete; not big enough to fall through, but the right size to twist an ankle if you’re not paying attention. These are spud holes, used to pin the roof in position. Think about the ballpark’s roof on a windy day. “It has the ability to act as a giant sail or parachute,” says Dave Wilke, senior director of facilities. The hydraulic pins keep it in place.

The Mariners also use this runway to launch fireworks after home runs, wins and on fireworks nights during the season. From here, the Mariners raise the team flags that are always arranged in order of division standings.

Next time you’re in the stands, look toward the bottom left corner of the big FireTV screen for a gray, unmarked room — that’s the control room for the roof.

You might think the controls for moving this massive structure would be a wall of knobs and switches, like an airplane cockpit. Nope. There’s a big green button marked “Go,” and a big red button marked “Normal Stop.” The emergency stop button, the equivalent of slamming on the brakes, is tucked under a plastic hood. “Once you get 12,000 tons moving, it wants to keep going,” Wilke says.

Opening and closing the roof is a three-person operation: one person on each runway and one in the control room, all communicating via walkie talkie. They make sure the rail is clear and nothing’s in the chains. From the control room, security cameras provide four views of each runway.

To move, the roof sidles along on 96 wheels, 48 on each side. “It’s a giant machine, so it pops and creaks as it moves,” Wilke says. At full speed, the roof moves at 50 feet per minute. It takes 15 minutes to open completely, and once retracted, the three panels nest together over the BNSF Railway tracks. (Fun fact: the roof can’t open during a game, only close.)

Wilke speculates that T-Mobile Park’s roof might be the largest moving structure west of the Mississippi. How much power does it take to move this 12,000-ton beast? Ninety-six 10-horsepower electric motors and 2,500 amps — that’s roughly 150 houses’ worth of power. The power and control cables that run along the roof track are like giant extension cords, the thickness of a wrist. And just like your house has downspouts from the roof, T-Mobile Park’s has downspouts, too, only its are 8-inch diameter pipes.

A repaint job, done in 2021 and 2022, cost just under $1 million. Reroofing the stadium would cost close to $4 million. “It is the single most expensive asset in the building, without a doubt,” Wilke says.