The Neptune Project is a high-voltage direct current (HVDC) undersea and underground power cable that links the PJM grid to New York, running approximately 65 miles between Sayreville, New Jersey, and New Cassel (North Hempstead) on Long Island, serving the Long Island Power Authority (LIPA) with enough power for 600,000 homes. It was developed, permitted, financed, constructed and is now operating the Neptune Regional Transmission System (Neptune RTS).

Neptune was completed in June 2007, on budget and ahead of schedule, by a consortium of Siemens Power Transmission & Distribution and Prysmian Cables and Systems under contract to Neptune RTS.  Since commencing operation, the system has exceeded expectations in terms of availability and the amount of power brought onto Long Island.  The project includes two converter stations, an undersea cable that extends approximately 50 miles underneath the Raritan River and the Atlantic Ocean, and a land-based underground cable that extends up the shoulder of the Wantagh Parkway on Long Island.  Technology, equipment, and installation methodologies used for Neptune will be essentially similar to those used for the Hudson Transmission Project.