The Big Dig, Boston’s massive underground highway tunnel system, is leaking water to the tune of 26 million gallons a year, The Boston Globe reported.

Engineers quoted by the newspaper say the repairs could take 10 years and involve lane closures.

The tunnel system was designed to handle 500,000 gallons of water removal a year – about 2 percent of the actual amount removed since last December.

Engineers also found documents that the firms in charge of construction may have known about the leaks as early as the late 1990s, The Globe reported.

The Big Dig was the largest construction project of its kind in U.S. history. It was created to take the elevated interstates that once ran into the center of Boston and replace them with wider, higher-capacity underground highways, including new tunnels and bridges to carry traffic over and under the city’s waterways. The Big Dig carries portions of several highways, including Interstate 93 and I-90.

Local media outlets reported in January that one of the tunnels had experienced icing problems that closed lanes in some areas. The Globe report linked those troubles to the leaks.