It's been rising in the Great Hall of the Pension Building for more than a month now -- soaring and jagging and cubing and tilting itself upward, all these improbable hanging boxes and layered aluminum towers and 1,000-pound folded galvanized forms piled precariously atop each other, each bouncing their weird and spectacular polygons of light.

The Thing, which doesn't really have a name, and until yesterday was surrounded by scaffolding and rigging ropes and cherry pickers and scissor lifts and the world's tallest stepladders, was supposed to have been finished by today at noon, for the grand unveiling and opening ceremonies. But it won't be. And maybe it doesn't matter. It'll be finished soon enough.

It is the sheer grandeur that's important, the awful daring. It looks bigger than the Statue of Liberty. It is sheathed in mirrored brass and 16-ounce copper and 26-gauge terne-coated steel. It is the vision of a postmodernist architectural visionary from Venice, Calif., named Frank O. Gehry, and what it is about, essentially, is celebrating a group of distinctly unpostmodernist Americans who work with their hands.

They are all brothers and sisters of the Sheet Metal Workers' International Association, and the rest of this story belongs to them, not to the architect whose head it has come out of. Because today, at the National Building Museum, which is what the Pension Building is called now, these 148,000 hard-hatted Americans, this year celebrating the centennial of their union, are going to have their long-in-coming moment in the sun. Gehry's tin colossus will be introduced to Washington as the centerpiece of a seven-month exhibit titled "Sheet Metal Craftsmanship: Progress in Building."

And the ones who have done the building in these last 40 or so days and nights -- all the skilled cutting and layering and snipping and fitting and dressing of these acres of gleaming metal, all the man-hours volunteered -- are the "tin knockers" themselves.

Tin knocker is a name sheet metal workers call themselves. Just don't you call them that.

Listen to one of them try to explain The Thing. His name is Mike Little and he's out of Local 63 in Springfield, Mass. Little, who has a way with profanity and hammers, drove in to Washington a couple days ago with his partner Biggie -- seven hours down the turnpikes, pit stop for a ham sandwich and a fill-up. Little, who has 22 years in his trade, has never seen anything like this structure, never dreamed he'd ever cut metal on something so strange. Hanging out of his various pockets are hammers and shears. On the back of his black T-shirt are the words "WE HANG DUCTS."

And he says:

"I've done copper, aluminum, stainless steel, galvanized iron, black iron, brass. Usually, we finish a job, the only thing anybody can see we've done is the grilles in the ceiling. All our duct work is hidden. That's the point, I know. But nobody can see how hard we worked, what kind of skills were needed for the job. Well, this thing is right out there. It says to the world: 'Hey, sheet metal is a very hard job. It must take some damn skilled work to put up something like this.' They called me and Biggie up couple days ago, they said, 'We got a big job going on down here, it's architectural steel, can you guys come down right away, we need you.'

"'Hell, yeah, we'll be down,' we said. See, sheet metal work is an expression through the hands and the mind. You got to know about algebra, about geometry. Maybe that sounds a little pretty to you. Here's the point. You just can't walk in off the street and get a pair of snips and a tinner's hammer and say, 'Hey, okay, I'm a goddam tin knocker, put me on the job.' No way. This thing you're looking at -- you call it art, I don't know what the hell I'll call it, but I'm beginning to like it -- tells the world: 'We're some proud guys. Maybe we don't have a sheepskin like you've got, buddy, but we got some other things. In fact, this thing you're looking at, it's our goddam sheepskin.' "

Mike Little's partner Biggie has been listening to this. Biggie, whose real name is Eddie Bigelow, and who has muscles like Popeye, and who also is nifty with epithets, is the president of Local 63. (Covers all of central and western Massachusetts, plus Vermont -- about 500 tin knockers.) Biggie, who's 50 now and who's been doing this work since he was 19, ever since the twins came, says: "Know what? I got 32 years in, and I'm finding myself talking about my apprenticeship. How do you explain that? This structure has taken hold of me, that's all I can say. Hell, I'd have come here for nothing."

He has come for nothing, you might say. The union is paying for his lodging (Best Western, around the corner) and giving him and Mike $100 each per diem. "Maybe that sounds like a lot," Biggie says. "Breakfast this morning was eight bucks. You know what that costs at home? A buck-fifty."

But nobody's paying for their time. That's gratis. Nobody paid for their gas, either, which is why they didn't stop. That, and they wanted to get right on the job.

Biggie's got his name felt-tipped on his green hard hat. They've been working all weekend. "You do it like this," says Mike, taking up a long rectangular piece of anodized aluminum. "You get a piece of tin, you nail it in, you hook it in, you pull it over."

"Or you can notch it in," says Biggie.

"We build a lot of jobs in the bar afterwards," says Mike.

"A man who doesn't have a sense of spatial relationships can't be a sheet metal worker," says Biggie.

"Takes a four-year apprenticeship to be a tin knocker," says Mike.

"You see that Tyson-Holmes fight?" says Biggie. "Christ, that kid can punch."

"It's rugged. It's not a job for {unprintable}," says Mike. "I guess you can't print that."

"We don't pawk the cawr in the Hawvawd yawd," says Biggie, miming a studious Cambridge type. Then he roars.

"Yeah, winter takes its toll," says Mike. "We wear Damart underwear, just like the pro football players. We still freeze our you-know-what off."

"I'm going to be able to retire with dignity when I'm 62 -- if I make it that long," says Biggie. "Then it's, whatever -- golf, fishing, women. Hey, I'm kidding, I'm married."

By the way, you guys ever heard of Frank Gehry?

"The last Frank I heard of was Lloyd Wright," Mike says. "That's a joke."

"You see all these other guys walking around here?" says Biggie. He is motioning at several of his brothers, some in the middle distance, some 65 feet up, on the top of The Thing. "Well, at night, back at the hotel, I go right over to them sitting in the bar and stick out my hand and say, 'Biggie. Local 63. Springfield, Mass. Howdy, bro.' "

Think of men climbing around on the nose of Abe Lincoln when they sculpted Mount Rushmore -- that's a little what the scale feels like. Except that this wonder is being built indoors.

Think of the last 20 minutes of "Close Encounters of the Third Kind," when Spielberg's space machine came floating down -- that's a little how otherworldly it all feels, especially to see it in the middle of the night, when chalk-white floods bathed it in a kind of moonglow.

This past weekend, about 65 SMWIA card-carrying members and apprentices were working around the clock on The Thing. They had ridden in from San Francisco and Portland and Topeka and Denver and a lot of other places, too. Some represented Local 100, which is the Baltimore-Washington union. Several were women. Two were husband and wife, Cheryl and Jeff Lewis from Greenbelt. Almost all were working gratis. Some of them have been here since early January, routinely doing 10- and 12-hour shifts, six and seven days a week, seeing nothing of Washington but the hotel and the job site.

Their pickups, in the museum's parking lot, sported things on their windshields like: PROUD TO BE UNION.

And: IS THAT YOUR FACE, OR DID YOUR NECK JUST THROW UP?

Men in the middle of the night on Saturday, with ashes suspended impossibly at the tips of their cigarettes, were using 2-by-4s as pounding mallets to dress a piece of metal, their hammering making eerie solitary thuds.

"You can hear by the sound of the wood whether you've tamped it in right or not," said a man with "BE WISE. ORGANIZE" scrawled on his front.

A man at the very top of the structure late on Saturday afternoon was caught in silhouette against the glinting terne-coated steel. The day was dying -- but the work was fierce. The man's hard hat, in shadow, looked like a perfect bubble. The man himself seemed the scale of an ant.

Cases and cases of Coke, yards of bread and bologna in the museum's coffee room. They went four, five hours, they took 10 minutes.

Sheet metal workers put up the geodesic dome at the 1986 Vancouver World Exposition.

Sheet metal workers did the Gateway Arch in St. Louis.

Sheet metal workers did the art deco at the Waldorf Astoria and the Chrysler Building in Manhattan.

Giant kerosense-fired heaters kept the workers warm in the cavernous spaces of the Great Hall, which is one of America's largest display spaces.

The structure was flying two flags this past weekend: Canadian and U.S. The union had sent down a Canadian brother. His name is Roland VanDaele but no one at the site was calling him that. They were calling him Frenchie. Saturday, Frenchie had a yellow climber's harness on and was working with giant slabs of mirror muntz brass. He is a thin nervous man with a Vandyke. "I got here on the fifth of January," he said. "I arrived at 11 o'clock on the airplane at National Airport, and I was on the job at noon. I wanted to see Arlington National Cemetery, the White House, all these things I see on CBC. I have seen nothing. The job is more important."

A coworker came up then.

"Don't lie to 'em like you do to us, Frenchie."

"Hey, gimme a cigarette," Frenchie said.

Then he finished what he'd been saying. "This is a proud moment for me to come here and work on this thing. Especially since I am Canadian. They are building a new Canadian Embassy in Washington, you know. I'm ashamed -- you can use the right word, maybe 'disappointed' -- that our embassy is being built nonunion." He shook his head. "Terrible."

But he quickly brightened. "Unbelievable, this thing. What can I say? Call it a piece of art. I don't really understand art. I've worked in the trade 27 years. I've never seen anything like this. I never will again. What can I say? I put everything I had into it. I wake up, I put on my clothes, I come over. My wife is coming for the opening. I'll meet her at the plane and get back to the job."

At midnight Saturday, Frank Ulrich was still on the job. He had come on at 6 a.m. He is out of California and has been here since shortly after New Year's. The sleeves of his flannel shirt were rolled past the elbow. He is a big man, with a heavy walk and a slow easy smile.

"I could've done some other things with my life," he said. "I'm glad I stuck around in sheet metal to get to work on something like this."