How a drill named Hazel is digging a tunnel underneath Alexandria, Va.

Deep underground, a construction crew is hard at work. They are digging a two-mile-long tunnel to help keep millions of gallons of raw sewage out of nearby waterways. And they have a lot of help: a massive drill named “Hazel” that weighs 380 tons — heavier than two commercial airplanes.

This $615 million project is key to cleaning up the Potomac River. When heavy rain strikes a centuries-old neighborhood in Alexandria, Va., the city’s pipes can’t handle the volume. Storm water and sewage overflow and pollute the river. The new tunnel will collect the overflow and redirect it to a wastewater facility, where it will be treated before draining into the river.

More than 800 other cities across the United States have similar sewer systems with just one set of pipes for rainfall and human waste. Some of them date back to the Civil War. Pushed to clean them up, communities have been building vast tunnels like this one.

Hazel has already dug more than 4,000 feet, burrowing under a cemetery, by a Catholic school and streets on the edge of Old Town Alexandria. In coming months, the drill will go under the river itself. The mining must be complete by the start of next year.

The Washington Post went 100 feet below ground to share this engineering feat with readers.

To get Hazel below the surface, construction crews spent seven months digging this shaft.

Located next to the Alexandria wastewater facility, this is home base for workers and engineers. It has all kinds of machinery and materials needed to make things run: water, electricity, even WiFi.

A wooden statue of Saint Barbara, the patron saint of miners, watches over the workers. It’s tradition to name mining equipment after women. Hazel was named after Hazel Johnson, a Chicago activist known as the “mother” of the modern environmental justice movement.

A crane lifts the drill up, piece by piece.

These are tense moments for engineers. They have prepared for months. The chains must stay secure for Hazel to be placed precisely on the ground.

At one point, the workers went up and down in what they affectionally call “the cage,” which was carried by a crane, just like Hazel.

The descent takes no more than a few minutes. It never really fazes the workers. “The hardest part about this is just like any job: Getting up in the morning,” said Jay Warner, a worker who takes the cage down every day.

It’s a grimy gig, but it comes with well wishes from the people of Alexandria. “You might be a boring machine, but I think you’re amazing,” Anthony Cummings wrote on Hazel’s metal hull.

The machine mines 20 hours a day, five days a week. On a good day, it drills 120 feet. To keep up, engineers need to work in 10-hour shifts. But the drilling takes place around-the-clock when Hazel goes under a road or other sensitive places.

Most of the people working with Hazel have spent decades working with similar machines across the country. Faced with a tight deadline from state officials, they have never had to work as fast as they have here.

Inside the drill sits Hazel’s operator, Phil Birch, who moves from city to city as tunneling projects pop up.“I’ve always worked underground. I’ve never had a job on the surface,” Birch said in a video interview with The Post. “It’s what you get used to, innit?”

To help Birch, the machine sprays a foam that makes it easier to drill through the soil. Hazel is digging through Potomac clay, which is kind of like a very stiff Play-Doh. “It’s always a battle,” Birch said. “You can’t have the ground gettin’ sticky on you.”

As the machine drills, it also lays grout and pushes dirt out of the way. This far down, most of the soil is indistinguishable. “You could mine through a dinosaur and it’s all going to come out looking like toothpaste,” said Ryan Payne, an engineer overseeing the project.

This conveyer belt moves the soil back to the surface and spits it outside.

This part of the tunnel is already dug. Drops of water and brown gunk fall from the ceiling, and a thick liquid sits on the bottom of the curved floor.

The temperature is always the same, and this far down, weather is never an issue.

“It’s quite the environment to be in for 10 hours,” said Payne, the engineer. “Down here, the time can go by really fast.”

Still, the air is humid. It feels clammy. This valve helps fresh air fill the tunnel.

Coffee machine. Microwave. All the touches of home.

The machine works like a big inchworm: It drills while slowly advancing forward, and then workers lay the concrete. This is a concrete panel that will line the tunnel as Hazel moves ahead.

Nearly 150 feet underground, workers wear kneepads to crouch and crawl around the tunnel.

As Hazel mines, a few people sit behind the drill shaft atop of a cocoon of wires and metal.“It’s tight in here, huh? I didn’t realize the diameter of the tunnel when I signed up for this job,” joked Jesus Moreno, the project’s tunnel inspector. “Another two feet would have been a little more comfortable.”

Ethan Gritton, the heading engineer, tracks how fast Hazel is moving and helps troubleshoot any issues underground. This is his first job out of college.

At the end of every shift, the commute back out of the tunnel is just a few feet longer. Workers ride on “moon rover” vehicles back out to the shaft.

Eventually, Hazel will join them above ground. Once the drilling is complete, a crane will lift the machine out of a similar hole as the one it first went down.

Additional video courtesy of RiverRenew.

Editing by Julie Vitkovskaya, Neeti Upadhye and Ann Gerhart. Design and development by Emma Kumer. Map by Chiqui Esteban and Emma Kumer. Additional development by Garland Potts. Design editing by Christian Font. Videos by Hadley Green. Video editing by Neeti Upadhye. Copy editing by Anjelica Tan. Photography by Bill O’Leary and Ricky Carioti. Photo editing by Mark Miller.