When the Northfield Mountain pumped-storage hydroelectric plant went into commercial service in 1972, it was the largest facility of its kind in the world. Located in Northfield, Massachusetts, approximately five and one-half miles up the Connecticut River from Turners Falls Dam, the 1,124 megawatt plant is entirely underground and does not depend upon the natural flow of the river for operation. Utilizing energy that is generated at nuclear and the more efficient of the area's fossil plants, water from the lower reservoir is pumped to an upper reservoir during periods of low power demand. The water is stored in the upper reservoir and then, at times of high electric demand, is released down a 1,100-foot-long pressure shaft to power a turbine generator and continues to the lower reservoir where it is stored until it resumes its cycle to the upper reservoir.

The 20-mile stretch of the Connecticut River, extending from the dam at Turners Falls north to the Vernon Dam in Vermont, serves as the facility's lower reservoir.

The man-made 300-acre upper reservoir, 800 feet above the river, is capable of storing 5.6 billion gallons of water.

The underground powerhouse includes four large reversible turbines, each capable of pumping 27,000 gallons of water per second and generating 270,000 kilowatts of electricity. The powerhouse is accessible through a 2,500-foot-long tunnel. Seven hundred feet below the surface, the cavern is longer than a football field and higher than a ten-story building.

Engineering studies for Northfield Mountain were begun in October 1964. Preliminary site preparation started three years later with construction of dams, dikes, and tunnels under way in 1968 upon receipt of a Federal Power Commission license.

The plant's operation does not affect the river water temperature and is nonpolluting. Power from the plant is quickly available to help maintain system reliability in emergencies or to help meet peak power requirements of over 1.7 million electric customers.