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With an estimated 3.5 million bulbs glowing along the harbor in Duluth, the Bentleyville Tour of Lights had already billed itself as “America’s largest free walk-through lighting display.”

But it’s about to get even bigger.

This year’s display, which opens Saturday, will shine with a million more lights than last year — 4.5 million total.

It’s come a long way from where it started in a local business owner’s yard in 2001. Creator Nathan Bentley is continuing to plan and organize the ever-expanding display, he said, mostly for the enjoyment of the kids who come to see it.

“There’s a lot of stress getting it open every year,” Bentley said. “Once it’s open, then it makes it worth its while. People come through and they talk about how amazing it is.”

This year, the tour’s 128-foot centerpiece “tree” will have 150,000 LED lights, up from 100,000 a year ago. Instead of glowing blue and white, as it has for the past few years, the tree will light up in red and green.

Visitors will also walk through more lighted tunnels and see new Duluth-themed displays and an expanded display thanking the country’s military troops. A chance to visit Santa will now come at the end of the walk-through display instead of in the middle.

Volunteers will still serve free cocoa, cookies, popcorn and marshmallows for roasting over one of the display’s many fire pits. Children 10 and under will still get a free knit hat.

Bentley started those traditions over years of putting up increasingly elaborate light shows at his house, first in Esko and later in rural Cloquet. As his displays grew, so did the crowds coming to see them.

Traffic and parking became a problem, and in 2008 Bentley decided to take a year off and re-evaluate. That fall, Don Ness, who was then mayor of Duluth, proposed moving the display to the city’s Bayfront Festival Park. With 20 acres of open space and parking nearby, there was a lot of room to expand.

It became a boon to the city.

Bentleyville is now a nonprofit with a $450,000 annual budget funded through donations and sponsorships. About 1,500 volunteers help set up the display and work at the site. Nearly 270,000 people visited last year.

Bentley said the display hosts families with children roasting marshmallows for the first time in their lives, families that travel up to five hours, couples on first dates and couples getting engaged.

Keeping it free of charge is a priority.

“We want everyone, whether you’re rich or you’re of little means, to have the same experience,” said Tim Rogentine, Bentleyville’s event coordinator. “We want it to be as accessible to everyone as possible.”

The display has an estimated economic impact of $20 million for the city — money that is spent on hotel rooms, restaurant meals and other purchases. Roughly 60 percent of visitors drive more than 50 miles to see the display, Rogentine said.

It’s been such a boost to Duluth that the City Council agreed to cover the cost of electricity this year, estimated at around $12,000.

Duluth Mayor Emily Larson called that a great value for something that brings so much energy and activity to town.

“It’s just so picturesque. It’s right on the lake. You’ve got the lift bridge right there. And then you get there and it’s free,” Larson said. “The community loves this event … It’s hard not to feel joy when you’re down there.”

Bentleyville will be open this year from 5-9 p.m. Sundays through Thursdays and from 5-10 p.m. on Fridays and Saturdays through Dec. 26. Donations are accepted. Visitors are encouraged to bring a new unwrapped toy or nonperishable food item for the Salvation Army.

@pamlouwagie • 612-673-7102