Look up! Plenty of buildings in New York City have more than 50 stories. Now look down. More than 50 stories under the city, an aging tunnel carries half the city’s water. And it has sprung some serious leaks. Grab your hard hat. We’re going in!

Workers travel deep under the Hudson River to get to the Delaware Aqueduct. The tunnel starts in the Catskill Mountains. It carries the city’s water supply over 85 miles. The tunnel’s weak spot formed where it crosses under the Hudson River. The rock there is limestone. Water leaks through easily. About 18 million gallons escape every day. That’s more than just a dribble. It would fill 27 Olympic-size swimming pools!

Workers can’t fix the gigantic leak. So they’re going around it. They use a tool nicknamed Nora. Nora is a borer (hole-digger). She’s about as big as a space shuttle. And she has sharp “teeth”—41 spinning, steel blades. Nora carves a new path through solid rock. Dozens of workers go deep down at a time. They work Nora's controls. They bolt together cylinders of steel, making the tunnel longer and longer. Workers slosh through puddles. They drive rail cars and tend to material going up and down the 900-foot shaft. It's a construction site in a wet cave.