After years of intense planning, San Francisco's Municipal Transportation Agency is scheduled to begin work on the tunnel for the controversial Central Subway project.

Construction of the 1.7 mile extension of Muni's T-Third line from SoMa, under Union Square and into Chinatown will begin on Tuesday with the creation of a "launch box" underneath the intersection of 4th and Brannan streets, where two enormous tunnel boring machines will eventually be inserted into the ground.

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This area isn't the only one to be affected by Central Subway tunnel construction. A flurry of activity is planned all along the 4th Street and Stockton Street corridors. Last week, work on 100 foot headwalls began near the Yerba Buena Center for the Arts at Folosom Street and the Moscone Center at Mission Street.

Over the course of the next year, further above-ground work will occur at Union Square and Washington Square Park. While the train itself will only travel as far as Chinatown, the machines are being removed from the ground all the way in North Beach and will have to burrow about six blocks past where the subway officially stops in order to get there.

Needless to say, this construction will have a detrimental effect on transit in the area. The San Francisco Chronicle reports:

Of course, several Muni lines and streets will be affected by the construction. The 30-Stockton and 45-Union/Stockton lines will continue the new routes they moved to in January; the 8X/8AX/8BX-Bayshore Express and 91-Owl will be re-routed in July, and the temporary 8 shuttle route will be eliminated. Passengers on the 38/38L-Geary routes should be prepared for delays in the Union Square area. During construction of the launch box, at least two lanes on Fourth Street will stay open at all times, which, of course, means that two will often be closed. All ramps to Interstate 80 will remain open during the day but could be closed at night. Needless to say, delays are expected and drivers are advised to steer clear of the area. Parking restrictions will also be in effect, and the sidewalk will be closed on one side of the street at a time.

Traffic issues are only expected to stem from work occurring on the surface. The massive tunnel boring machines are reportedly undetectable while operating underground. "There's no vibration, no surface disruption," Central Subway Project Manager John Funghi told the Chronicle earlier this year. "You could be walking on top of it and not even feel it."

Check out this Google Map complied by SFMTA showing all of the detours, disruptions and Muni rerouting caused by the construction.

The Central Subway project has been plagued with controversy virtually since its inception, with critics deeming its $1.6 billion price tag as too hefty for the level of ridership the line will likely end up carrying.

A civil grand jury report on the project issued last year, entitled Central Subway: Too Much Money For Too Little Benefit, recommended overhauling the subway's design to decrease costs improve efficiency. A 2007 report by former Muni Planning Director Tom Matoff found that the subway "does not...meet the market needs of the corridor it is intended to serve," and that it "might actually worsen travel conditions for some customers, without a compensating improvement."

However, SFTMA attests that not only will the Central Subway become the most-used line in the city's entire transit system by 2030, but it will also have the second-highest rate of daily boardings per mile of any light rail line in the entire country.

During last year's mayoral race, opponents of then-interim mayor Ed Lee attempted to use his vocal backing of the subway as a campaign issue. City Attorney Dennis Herrera, for example, took the opportunity to go back on his previous support for the project and take Lee to task for continuing to advocate for the plan even though its cost had more than doubled from an initial estimate.

Despite the opposition, Lee never wavered in his support of both the project and the $942 million in federal grant money it is expected to bring into the city.

Washington isn't the only place where local leaders are looking to for a hand picking up the subway's considerable tab. SFMTA has been working to secure $61 million in state funds from California's equally controversial High Speed Rail project in addition to the $48 million Sacramento already kicked in last week.

"The Central Subway will cut peak-period travel times in half compared to current transit options, easing travel through the busy 4th Street and Stockton Street corridors," SFMTA chief Ed Reiskin said in a statement. "Our partners in Sacramento have consistently recognized that these projects are key to improving transportation in San Francisco now and for future generations."

The tunneling machines are expected to drop into the ground and get to work sometime next year.

Check out this slideshow of images from SFMTA showing what the Central Subway will look like: