For 14 years, it was a tunnel to nowhere, essentially an $8 million, 1,000-foot-long concrete box buried quietly beneath a taxiway at Salt Lake City International Airport.

But the current $3.6 billion project to replace airport facilities “really couldn’t be built without this tunnel being in place,” Mike Williams, manager of the project, said as he showed the reopened tunnel to the news media for the first time Monday.

“It showed some amazing foresight by past leaders,” Airport Director Bill Wyatt has said about that tunnel, adding that it saved tens of millions of dollars and allowed moving around puzzle pieces to make the overall project possible at the same time that air operations continue.

The tunnel will connect two new parallel concourses at the airport, passing beneath an airplane taxiway. It was built in 2004, after 1990s master plans envisioned airport renovation — but construction was still a decade away.

At that time, the airport “was doing some construction on the airfield. Rather than coming in later and tearing all that up again, they built the tunnel at that time,” Williams said. Some federal funding for it was also available then.

The tunnel cost about $8 million, officials say, but would cost many times that now.

For example, just the construction of facilities to connect the tunnel to the new concourses — including digging, concrete pours and installation of escalators, stairs and utilities — is now costing more than $19 million.

Eventually, the renovated airport also will have another main, central tunnel to connect the parallel concourses. That is scheduled for a later, second phase of construction because the current Concourse D used by Delta Air Lines would need to be demolished first.

The tunnel built long ago helped the airport to develop a combination of enough old and new airline gates available through various construction phases to allow air operations to continue uninterrupted.

Williams described the challenge as, “We’re building our new house on top of our old house,” and trying to live in it at the same time.

Opening of the first phase of the reconstruction project now is about 700 days away, in fall 2020, and airport officials gave news reporters a tour Monday of the progress.

“We are on budget and on schedule for the first-phase opening,” Williams said. More than 1,800 construction workers are laboring daily on the project.

Williams pointed out some features designed to add comfort. All new chairs will have electric plugs to allow passengers to recharge their electronics. Bathrooms are spaced every 300 feet — and those for women are larger and have more facilities, including rooms for nursing mothers, than those for men.

The new terminal includes a special area for crowds to greet returning Latter-day Saint missionaries or soldiers.

“I call that the Salt Lake phenomenon,” Williams said.

That reception area will include concessions, a large fireplace, seating for 300 to 400 people — and a replica of the world map now found on the floor of the existing Terminal 1.

The new terminal will include 16 security lines. Instead of people lining up one by one for those lines, a new design will allow five at a time to fill bins with belongings and proceed according to whoever is ready first. Williams said it should speed screening.

The current airport is handling 26 million passengers a year, more than double its designed capacity — using 71 gates (55 have jet bridges).

The redesigned facility will have 78 gates, all with jet bridges (and more seating than exists now). The design also prevents current problems where some jets can block in others, slowing operations. It is also designed to allow easy expansion in the future.

The new terminal will sport artwork and designs reminiscent of Utah’s canyon country. Williams said early surveys showed that residents “wanted to bring the beauty of Utah into the facility,” so instead of a terminal with traditional colors of white and gray, “ours is copper colored” with warm colors elsewhere.

The new airport will include a garage that is twice as large as the current one, with 3,600 parking stalls. Car rentals will take the first floor of the parking garage and have 1,200 stalls there for rental operations.

The new airport will also have an 2.1-mile elevated roadway to drop off departing passengers at a different level than arriving passengers.