Lengths of each tunnel has been given. With knowing their diameter one can not appreciate their enormous size.
Now that we know it was an eight-inch steel pipe that brought the world's largest-diameter tunneling machine to a halt up in Seattle, we can no longer fantasize about Bertha unearthing a five-story-tall buried locomotive. But we can look at something almost as fascinating: The other giant holes that are currently being churned through the bedrock below our cities.
With major infrastructure projects underway across the country, chances are very good that something is being hollowed out below your feet at this very moment. Here are some of the largest tunnels currently under construction in the U.S.
Until being halted by a steel wellwater casing, the tunnel boring machine (TBM) named Bertha was digging a two-mile tunnel for the Alaskan Way Viaduct Replacement Program, repairing a double-decker highway along the Seattle waterfront that was damaged in the 2001 earthquake. In late December, contractors began drilling wells around Bertha, removing more than 300,000 gallons of water to lower the pressure around the machine so crews could get a closer look at what we'll now call "The Pipe." The project is still scheduled to open in late 2015.
That machine you see there is Mom Chung, the adorable TBM that's working on the Central Subway, a $1.5 billion light-rail extension that's the first new subway route for the city's municipal railway (MUNI) in 50 years. Along with Mom Chung's trusty sidekick Big Alma (Chung on southbound, Alma on northbound), the two machines will carve out 1.7 miles of twin tunnels below the existing MUNI and BART lines. The three underground stations and one above-ground station on the route from South of Market to Chinatown, will bring rail service to one of the most densely populated areas in the country. The new line is planned to open in 2019.
New York City
One of the largest tunnels in the world, City Water Tunnel No. 3, has been under construction for 43 years—43 years! —at a cost of $5 billion. In total, the tunnel is 60 miles long, and at some points travels 500 feet below the ground. The second phase of the tunnel, which supplies drinking water to Manhattan, opened in 2013, with the completion of this cathedral-like space under Central Park, a valve chamber for the system. The Brooklyn-Queens portion of the tunnel will be completed by 2018, ending New York's largest construction project to date.
Deep below Lake Mead, the Southern Nevada Water Authority is digging a tunnel to its newest "Intake No. 3"—basically, a $800 million straw that will help suck the water out of the lake to augment the municipal water supply, even when lake levels are low. One of the largest and most complicated water projects in the country, the tunnel is three miles long through solid rock. The project has been plagued with problems due to the extremely "unpredictable" geology; in 2010 workers hit a fault zone which flooded the tunnel with water and debris and required them to change course. It should be finished in May of this year.
A TBM named Mackenzie finished her job at the Euclid Creek Tunnel in 2013, digging a three-mile tunnel along the shoreline of Lake Erie. It's one of seven tunnels that are part of Project Clean Lake, which will help mitigate pollution in local waterways by storing up to 60.5 million gallons of stormwater and wastewater. The 25-year, $3 billion project will help bring the city in compliance with the Clean Water Act. It should be completed by 2035.
Crews have begun laying the roadways inside the Port of Miami Tunnel, which will zip cars from the city's port to local freeways without having to travel on surface streets through downtown. Two 4,200-foot tunnels travel below Biscayne Bay, but because of the complex geology—soft, porous limestone—a total of 18 miles of drilling were required. Most notably, in the instance of a hurricane, two 50-ton metal gates will seal off the tunnel, preventing flooding. The tunnels will open this spring.
New York City
The long-anticipated Second Avenue Subway has been underway, in some form, since 1929 and is the first subway line built beneath Manhattan since 1932. Phase 1 of the project, which travels south from 96th to 63rd Street on the Upper East Side, is underway and expected to be completed by late 2016. One of the largest excavations, the 86th Street Station Cavern (and yes, it is called that) was finished over the summer, and now crews are working to waterproof the tunnel. That's only one small part of the 8.5 miles of tunnel proposed to complete the project and connect the east side of Manhattan to the subway system.
The Clean Rivers Project in the DC area is currently at work on the 13-mile-long Anacostia River Tunnel which will capture and store post-storm sewer overflow that's currently being discharged into the Anacostia and Potomac Rivers. A TBM named Lady Bird (after the environmentally concerned first lady) is currently digging the first four-mile portion, known as the Blue Plains Tunnel. Since being lowered into service in June, Lady Bird is currently tunneling about 100 feet a day, and soon a second (yet-unnamed) TBM will begin tunneling at the far end of the project, meeting Lady Bird at the middle. The Blue Plains Tunnel will be complete in 2015 with a system of smaller tunnels and sewers installed to help divert the flow. The entire project should be done by 2025.