New York City's sewer system is one of the oldest and most complex in the world. The first sewers were built in the 1850s, and the system has been continuously expanded and upgraded ever since. Today, NYC has over 7,500 miles of sewer lines serving 8.3 million residents. The system includes combined sewers (which carry both stormwater and sewage), separate storm sewers, and separate sanitary sewers. Many of the oldest pipes in Manhattan and Brooklyn date back over 100 years and are still in service. The NYC DEP manages the system and has invested billions in upgrades, including green infrastructure to reduce combined sewer overflows.