Twenty-six-foot-tall sound barriers surround the construction site between White Rock Creek and the Stonewood Terrace Apartments on Scyene Road, near Dixon Avenue. An armed guard secures the entrance. You would never know from the outside that behind those walls, and deep beneath our feet, lies the starting line of one of the most expensive projects in this city's recent history.

This project, the Mill Creek Drainage Relief Tunnel, is also one of Dallas' most important projects. And, easily, its most breathtaking.

By the time the clock strikes 2023, City Hall will have spent more than $300 million in bond dollars to protect billions of dollars' worth of properties — some of them the city's most expensive, most essential. The tunnel will run for 5 miles. It will be 35 feet in diameter. It is being dug 120 feet below this city's surface.

Its project managers said over and over Thursday it's the largest tunnel being dug in this country at this moment.

The Mill Creek project has also given us a rare glimpse at Dallas' prehistory, in the wall, 40 feet tall and wide, of millions-year-old rock serving as the project's starting line. This wall — what the contractors call the tunnel's "face" — reveals fractures that geologists believe were created hundreds of thousands of years ago.

"Healed fault lines, they're called" said Rachel Sackett of Southland Holdings LLC, the parent of the Roanoke contractor digging this tunnel. "Because they look like scars."

This out-of-sight project has been in the works for eons. Dallas Water Utilities' Milton Brooks has been in charge of the Mill Creek Drainage Relief Tunnel since 2007. Voters approved bond funding for it in 2003, 2006 and 2012. But help is at last on the way for neighborhoods that have pleaded for reprieve from floodwaters for decades.

"I'm gonna get it done," Brooks said.

Sometime in August, a 3-million-pound machine will go to work, burrowing through bedrock as it begins its journey from South Dallas to a parking lot next to Sammy's Texas Barbecue in Uptown. Those living atop its path — which begins along Scyene Road and Dixon Avenue and winds through Fair Park, Deep Ellum, Uptown, East Dallas and points between — will never have a clue about what is happening in the ground below.

Only later, in four years or so, will those people realize the work done in the far-below — when their streets stop flooding, when their homes and businesses and cars are no longer underwater in the storms, when a prolonged downpour is no longer something to fear.

1 / 4The Mill Creek Drainage Relief Tunnel in Dallas on Thursday, June 13, 2019. Once completed, the 5-mile tunnel will sit 70 to 100 feet below ground to provide flood relief. (Shaban Athuman / Staff Photographer) 2 / 4Lee Kleinman (from left), the Dallas City Council member who chairs the transportation and infrastructure committee, visits the tunnel with council colleagues Carolyn King Arnold and Adam Medrano and councilman-elect David Blewett. Behind them is a wall of millions-year-old sediment.(Shaban Athuman / Staff Photographer) 3 / 4Dallas City Council member Adam Medrano takes a picture of Carolyn King Arnold as she touches the fault line during a visit into the Mill Creek Drainage Relief Tunnel.(Shaban Athuman / Staff Photographer) 4 / 4Tim Win walks from one end during a visit into the Mill Creek Drainage Relief Tunnel in Dallas. Once completed, the 5-mile tunnel will sit 70 to 100 feet below ground to provide flood relief.(Shaban Athuman / Staff Photographer)

To drive the route above the surface takes 12 minutes, at most, in mid-morning traffic. But this underground journey will take 12 months, maybe as many as 18, as the tunnel-boring machine gnaws through limestone that crumbles to the touch but is only slightly softer than concrete. And even after the boring machine reaches its final destination, this project will be far from complete. Several intakes must be dug between South Dallas and Uptown, hence the estimated timeline for completion.

Lee Kleinman, chair of the Dallas City Council's Mobility Solutions, Infrastructure and Sustainability Committee, called a special meeting Thursday to tour the tunnel. Only three of his colleagues — Adam Medrano, Carolyn King Arnold and newcomer Dave Blewett — accepted the invite; those who skipped will likely never have another chance.

Kleinman had already toured the tunnel, but asked for another excursion so colleagues could get an up-close look at a project that so bedeviled City Hall for years. The North Dallas council member, a native of this city, is enamored of such treks to the far-below.

"The best thing about being on the Mobility Solutions committee are the tours," Kleinman told Medrano as we sat around the table, awaiting safety instructions from contractors-turned-temporary guides. "I love tours."

Five years ago, on a cold Saturday morning, Kleinman got city workers to show him the East Bank-West Bank Interceptor Connection being dug beneath the Trinity River. That's some 70 feet below the surface — a tunnel about 12 feet wide, 3,300 feet long that carries some 120 million gallons of waste each day to a wastewater treatment plant and the river. That tunnel is a plastic straw in a soda can compared to the Mill Creek project.

Mike Seely of Southland warned everyone going on the tour: "It's a working site." He explained the giant white rocks all over the site were meant to keep it from getting muddy, unworkable. He said, too, that the Austin Chalk "gets a little slick," and instructed us to be careful not to slip once we exited the "man cage." He took a brief pause and renamed it: "personnel basket."

Prepped and dressed, in steel-toed boots and construction hardhats and bright yellow vests and safety googles, we walked the rocky terrain to the site. Kleinman peered over the lip of the entrance into the cavern carved to make room for the tunnel-boring machine: "That's a big [expletive] hole," he said.

Kent Vest, a Southland project manager, pointed toward the boring machine workers were assembling on site. He noted that it looks a bit like a spaceship.

"Everything I know about drilling tunnels comes from Ocean's Thirteen," I said.

"Everyone says that," said Vest.

This was a nice field trip, a thrilling diversion that turned adults into giddy schoolchildren for a morning, but this tunnel has been long needed. And getting here was no easy thing.

1 / 3Peeking into the entrance to the drainage tunnel, which lies 120 feet below.(Shaban Athuman / Staff Photographer) 2 / 3Tim Win, a Southland Mole director, and the "face" of the tunnel, which spans some 40 feet high and wide.(Shaban Athuman / Staff Photographer) 3 / 3Dallas City Council member Lee Kleinman (left) and project director Milton Brooks look at the tunnel-boring machine that, when completed, will weigh about 3 million pounds and spend the next 12 to 18 months tunneling beneath Dallas.(Shaban Athuman / Staff Photographer)

Perhaps you do not recall, or ignored the story because it involved the City Council and audits and bids and other boring things, but in 2016 the council awarded the contract, worth more than $200 million, to Southland Mole. Shortly thereafter, then-City Auditor Craig Kinton issued a report saying the process was mismanaged after the lowest bidder, an American offshoot of a Brazilian company embroiled in a massive corruption scandal, was disqualified. After a do-over, Southland emerged again as the winning bidder last year.

"This project means a lot of relief for a lot of people," said Dallas Water Utilities Director Terry Lowery while we waited for our spin in the cage. "It's not sexy, and most people will never know it's there, but the impact is huge."

For now, and for far too long, this eastern slice of the city lies in the 100-year floodplain. The drainage relief tunnel will shrink that map considerably. The city never has determined exactly how many people the tunnel will take out of the floodplain. But officials say the tunnel will help slightly more than 6,000 properties, worth a combined $4 billion, including Baylor University Medical Center, where steady and heavy rains threaten the emergency rooms and trauma center.

1 / 2 2 / 2The before and after looks at the flood plain after the tunnel is completed(City of Dallas)

"It's a sizable chunk of the city," said Lowery as she climbed into the basket to take her below.

Once underground, the council members and city staffers posed for photos along the tunnel's face, and ran their hands along the fault lines — awed, like children. Arnold shouted wooooooo several times, to hear the echo through the human-made cavern.

"This is so neat," she said through a wide, wide grin.

Neat, yes. And needed, desperately.