they first went to the hudson highlands, but 150 years later, it went to the delaware highlands. really diverted the water that normally went to philadelphia to new york city. i don't think they anticipated that. narrator: the majority of new york city's drinking water comes from watersheds in upstate new york. a watershed is the area of land where water from rain or snow melt drains downhill into a body of water. mountains act as a funnel to feed rivers and lakes. and in this case, reservoirs. in the new york city system, water is collected and stored in 19 reservoirs, which can hold more than a year's supply -- over 580 billion gallons of water. almost all of the system is fed by gravity, without the use of energy-consuming pumps. valves open to regulate the flow into the 85-mile-long delaware aqueduct -- the longest tunnel in the world. at hillview reservoir... the water is partitioned into another giant tunnel system. where it travels deep below manhattan. the pressure built up by gravity from the mountains pushes the water upwards toward the surface through vertical shafts.