New York City is supplied with drinking water by the protected Catskill Mountains watershed. As a result of the watershed's integrity and undisturbed natural water filtration system, New York is one of only four major cities in the United States the majority of whose drinking water is pure enough not to require purification by water treatment plants. The Croton Watershed north of the city is undergoing construction of a US$3.2 billion water purification plant to augment New York City's water supply by an estimated 290 million gallons daily, representing a greater than 20% addition to the city's current availability of water. The ongoing expansion of New York City Water Tunnel No. 3, an integral part of the New York City water supply system, is the largest capital construction project in the city's history.