Steve McQueen's famous "Bullitt" Mustang will hit the auction block

The famous muscle car that Steve McQueen, the "King of Cool," drove in the classic 1968 movie "Bullitt" gave birth to the modern-day car chase scene. The 1968 Mustang GT became a legend — especially after it disappeared for decades. 

Now, it's resurfaced in Kissimmee, Florida, and is up for auction today. The rusted, dented highland green 1968 Ford Mustang GT Fastback is worth a fortune. The Mustang is expected to go for millions, CBS News correspondent Mark Strassmann reports. 

"This is the holy grail of muscle cars. It's the holy grail of Mustangs," said Dana Mecum, founder and president of Mecum Auctions, the world's leading auction house for collector cars.

"This will pretty much be the first American muscle car to sell as art," Mecum said. "It's gonna sell as pop culture."

Through a 10-minute thrill ride on the streets of San Francisco in "Bullitt," McQueen and his classic pony car became the epitome of old-school cool and reinvented chase scenes for Hollywood. But the car's disappearance after the movie led collectors on their own chase. 

"It was never lost. I mean, it was always in the garage," said Sean Kiernan, 38, whose father bought the Mustang in 1974. According to Kiernan, his dad found the car on a back page ad on "the Craigslist of the day."

The ad read: "Driven by Steve McQueen in the movie 'Bullett' [sic] ... Best offer." Robert Kiernan paid $3,500. 

"He's the only one who showed up," his son said. "Paid for it, took off. And it ended up in the driveway." 

Robbie Kiernan, Sean's mother, drove the car every day to her job as a third-grade teacher in New Jersey.

"Back and forth to school," Robbie said. "It was just, you know, loud. I would have to drive through the center of Madison. And they all knew the car… 'Here she comes.'" 

She was driving a hugely valuable collector's item, but didn't know it.

"It was, to me, a used car," she said.

In 1977, McQueen tracked down the Kiernans and wrote to them. He wanted to buy what he called "my 68' Mustang." They never answered. In 1980,  its clutch died. The car was parked in a barn for 35 years. 

"The car sat on jack stands," Kiernan said. "And the world kind of evolved around it."

After his father died in 2014, Sean, an admitted "car nut," completely restored it himself — 98% original parts. It still had the camera mount, a huge hole cut in the trunk for the smoke machine, and the bray of its 390 big block engine.

Over the last year and a half, Kiernan has shown off the car around the world as part of a promotion with Ford. He swore he'd never sell it — because he could still see his father inside it.

"Building and unveiling the car has been extremely therapeutic," he said. "The car had become — has become bigger than my life. And yeah, I want to go home. And I think the car deserves another good home."
 
For nostalgia's sake, Sean will start the bidding at $3,500, the price his father paid. But don't kid yourself, the car could go as high as $5 million dollars.