Here's what building Millennium Park looked like

Millennium Park is celebrating its 20th birthday this weekend, meaning many Chicagoans remember it before it transformed into the 24.5-acre gem it is today.

The big picture: One who remembers is Ben Campney, a former city employee who photographed the construction .

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Flashback: Before it was green space, a pavilion, a fountain and home of "The Bean," it was a big parking lot and old railyard.

Catch up fast: Millennium Park opened July 17, 2004 . Construction started in 1998 and includes designs by major artists and architects. Frank Gehry designed the Jay Pritzker Pavilion and BP Bridge; Anish Kapoor created Cloud Gate (aka The Bean) and Jaume Plensa designed Crown Fountain.

• With the parking garages below, Millennium Park became the largest green roof in the world.
• It was the dream project, and a crowning achievement, of Chicago's longest serving mayor, Richard M. Daley.

By the numbers: Building Millennium Park ended up costing $490 million, in a mix of public and private dollars .

• $270 million came from city parking revenue bonds and tax dollars.
• $220 million was from donors, foundations and corporations.

Zoom in: Campney worked in Chicago's Public Buildings Commission (PBC) and was sent to the construction site twice a month to document progress to show the mayor.

• "He loved Millennium Park," Campney tells Axios.

Photo: Courtesy of Ben Campney

• The only thing that's noticeably aged is the Jay Pritzker Pavilion, Campney tells Axios after his recent visit to the park.

Photo: Courtesy of Ben Campney

• The Chicago Department of Transportation (CDOT) and private contractors Harston/Schwendener started construction in 1998. Walsh Construction took over as general contractors, and PBC replaced CDOT in 2001.

Photo: Courtesy of Ben Campney

Campney's favorite photo is the one above — his first selfie at The Bean.

• "After a while, they tented it because they didn't want anyone to see it until it was done," he says.

Photo: Courtesy of Ben Campney

The intrigue: "What we [PBC] do is we build something, we cut the ribbon, and we move on to the next project. We're done with it, right? So in the case of Millennium, I needed server space. All the pictures were copied to CDs and tossed in a can on my desk, and I forgot about them for 20 years."

• Campney tells Axios he forgot until he saw our last story about The Bean and he dug up his old photos.

Go deeper: Check out more of Campney's photos of building Millennium Park.