They're water tunnels. They deliver water from the Reservoir to the New York Metro Area. They're so deep underground for two reasons (1) imagine you have to bui

They're water tunnels. They deliver water from the Reservoir to the New York Metro Area. They're so deep underground for two reasons (1) imagine you have to build a tunnel that should last for the next 150 years – it needs to survive nuclear war, new technologies you've never heard of, meteor strikes – any risk of contamination is unacceptable and (2) more importantly, because the terminals of the tunnels are so low with respect to the reservoir, 95% of the pressure needed to deliver water from the reservoir to all of the buildings in New York is provided by gravity. Only 5% of the pressure comes from pumps. Which is pretty amazing.