A German-made tunnel boring machine will dig the tunnel, which will be 23 feet in diameter.

There have been some big projects in D.C. in recent history, but nothing as substantial as the construction of the Metro. Well, today D.C. Water is kicking off what it’s calling the biggest single infrastructure project since that time.

D.C. officials gathered today to christen Lady Bird, a 400-foot-long, 1,300-ton German tunnel boring machine that will dig four miles worth of tunnel some 100 feet below the bottom of the Potomac River. Once the massive tunnel is done, it will form part of a network of similar repositories—the Clean Rivers Project—that will allow for the storage of runoff from the city’s aging combined sewers during heavy storms. Currently, that runoff—a gross mix of rain water and sewage—dumps into the Potomac and Anacostia Rivers.

According to D.C. Water, Lady Bird will start from two shafts at the Blue Plains Water Treatment Plant in Southwest D.C. and dig a Metro-sized tunnel heading north some 24,200 linear feet. Other such tunnels are planned for the Potomac and Rock Creek watersheds, and a small version of the tunnel is being dug out at the McMillan Sand Filtration Site, where it will help control flooding in Bloomingdale and LeDroit Park.

Beyond just the tunnels—which a number of other cities also have—D.C. has been pushing for more green roofs and surfaces that absorb water instead of allowing it to flow straight into sewers and the rivers.

And because it’s all about social media these days, Lady Bird even has a Twitter account where you can follow her progress.