The 20-inch (50cm) water main gave way under 40th Street and Seventh Avenue at 3 am.
The rushing water was only a few inches deep on the street, but videos posted on social media showed the flood cascading into the Times Square subway station down stairwells and through ventilation grates.
The water turned the trenches that carry the subway tracks into mini rivers and soaked train platforms.
The excavation left a big hole at the intersection of 40th Street and Seventh Avenue, where workers were digging with heavy equipment to get to the broken section of pipe.