BETHLEHEM, Pa. — Sixteen thousand tons of Bethlehem Steel collapsed in a matter of seconds Sunday as a demolition crew imploded Martin Tower, the defunct steelmaker’s former world headquarters.

Crowds gathered to watch the demolition of the Pennsylvania area’s tallest building, a 21-story monolith that opened at the height of Bethlehem Steel’s power and profitability but had stood vacant for a dozen years after America’s second-largest steelmaker went out of business.

Explosives took out Martin Tower’s steel supports and crumpled the 47-year-old building, which had earned a spot on the National Register of Historic Places despite its relatively young age. The implosion, which took 16 seconds, created a thick plume of dust that lingered for several minutes.

Tyler Kent, whose father worked at Bethlehem Steel for 46 years and raised 11 children, said his “heart stopped” as he watched the building fall. His father and other relatives took pride in working at the industrial behemoth that armed the U.S. military and helped shape skylines across the country.

“To see it come down brought a tear to my eye. I didn’t think it was going to affect me emotionally like it did, but I just can’t imagine it’s gone. It’s so sad,” said Kent, who could see the tower from his house.

Martin Tower’s current owners spent years trying to redevelop the 332-foot (101-meter) structure — the tallest in a heavily populated region of Pennsylvania that includes the cities of Allentown, Bethlehem and Easton — but ultimately concluded it made more economic sense to knock it down and start over. Plans call for a $200 million development with medical offices, retail stores, a restaurant, a convenience store, a hotel and 528 apartments.

Bethlehem Steel was a major supplier of ships and armaments to the US military during World War II, and its steel is found in the Empire State Building, the Golden Gate Bridge and many other landmarks.

The company moved into its new corporate headquarters in 1972, shortly before the US steel industry plunged into a severe recession. Bethlehem Steel, which employed more than 120,000 people when Martin Tower opened, declared bankruptcy in 2001 and closed for good two years later.

To some, the tower — built in a cruciform shape to maximize the number of corner offices — symbolized corporate excess.

“This is where the money went that the workers never got,” said Fran Maiatico, whose father worked at Bethlehem Steel. She was among hundreds of people who gathered several blocks away from the building Sunday to watch it come down.

Leonard Gentilcore, 88, a retired Bethlehem Steel structural draftsman who worked on Martin Tower, said he didn’t care that it was gone. He said he associated the building with out-of-touch company executives who helped drive Bethlehem Steel into the ground.

But his son, 49-year-old Mike Gentilcore, a former Bethlehem Steel metals researcher, said “it breaks my heart” that an important piece of the company’s history is no more. He recalled looking out the tower’s windows as a child, and later worked there himself.

“It’s the end of an era and I’m going to miss seeing it there,” he said.

The company’s flagship Bethlehem mill, less than 2 miles (3 kilometers) from Martin Tower, was redeveloped into a casino and entertainment destination 10 years ago.