Wiscasset is being considered for the largest energy development proposal - and potentially the largest development project of any kind - in the history of the state.

A Toronto entrepreneur who has developed Canadian wind farms has floated the idea of building a massive $2 billion underground hydropower station at the old Maine Yankee nuclear power station site.

The project would be one of the first of its kind anywhere.

The proposal raises questions about impacts on the Back River and groundwater, and it would use as much energy as it creates.

But local and state officials, as well as environmentalists and others who fought an earlier proposal for a $1.5 billion coal gasification plant at the site, said the idea has a lot of appeal because of its potential to create jobs and help develop the state's clean-energy infrastructure.

The plans presented by Riverbank Development Corp. in recent meetings here and on its Web site (www.riverbankpower. com) call for the construction of cavernous reservoirs and a three-story-tall power plant carved out of the bedrock 2,000 feet beneath the ground. At times of peak electricity demand, tidal water from the Back River would surge straight down four large chutes, through power- generating turbines and into the caverns, each of which would be 100 feet tall and 1,000 feet long.

Then, when electricity demand is low and there is excess power going into the grid, the water would be pumped back to the surface.Called an aquabank

The plant - called an Aquabank - would operate in six- to eight- hour bursts and generate 1,000 megawatts of power, more than Maine Yankee used to and more than all of Maine's hydro dams combined. It would make a profit by using power when the price is low and selling when it's high.

Although the system would use at least as much energy to pump out the underground reservoirs as it generates when filling them, experts say the power-storage system could help Maine make the shift from fossil fuels to cleaner renewable power sources.

"It's not adding new energy to the state of Maine," said Habib Dagher, a professor of engineering at the University of Maine and a proponent of offshore wind power. "But it's like buying a big battery for your house and storing energy in it. It's a huge battery."

There are pump-storage systems around the world that use surface reservoirs, but none below ground, he said.

The idea could be especially valuable because of the state's goal of developing 3,000 megawatts of wind energy by 2020, according to Dagher and others.