They recently signed a lease for an iron ore mine that has run out of iron ore. A hole in the ground. A hole in the ground they'd never seen. A hole in the ground that, by the way, is flooded by water a half-mile deep.

But even visionaries get nervous. "This is it?" Hoe asked Broome the other day as they stood atop a concrete slab covering the shaft of Bethlehem Steel's old Grace Mine. "There's a hole under here?"

Hoe and Broome don't care about extracting ore from the rolling Berks County countryside.

No. Thar's air in them thar hills.

Compressed air.

Hoe, a Philadelphia developer, and Broome, a Reading engineer, have formed a company that intends to pump the abandoned mine full of air by night, and then release the pressurized air during the day to drive turbines and generate electricity.

That may sound far-fetched, but an Alabama utility has been operating such a compressed air system for more than a year using a cavern bored out of a salt dome. Another plant has operated in Germany for 15 years.

The $75 million project might also be the first electrical generating station that doubles as a tourist attraction.

That's because the owner of the mine, developer Raymond H. Carr, has long-standing plans to build a Victorian resort and amusement park on the 4,200-acre property he bought eight years ago from Bethlehem Steel. His other ideas for the tract, which last year became the independent borough of New Morgan, include a housing complex, an industrial park and a garbage dump.

Carr did not anticipate renting or using the excavations, which have filled with water since Bethlehem stopped production in 1977.

"I thought that mine was something that was concreted shut to stay shut forever," he said.

But when Hoe and Broome approached him with their notion, Carr not only agreed to the lease, but also asked that the plant be included in any potential tourist attraction at the site, which lies 45 miles west of Philadelphia near the Morgantown interchange of the Pennsylvania Turnpike. "It's going to be the most exciting thing to happen on the East Coast," he said.

Compressed air systems are simply the latest method to store electrical energy that is generated at night, when rates are low, and use it during the day, when electrical demand and prices rise.

Some utilities, including Philadelphia Electric Co., operate water storage systems that employ the same principle: Water is pumped into a reservoir at night and then is released during the day to run generators. Other utilities are building enormous batteries to store electricity generated at night.

LIKE A SCUBA TANK

Hoe and Broome, who have collaborated on other energy projects, say their system would be the first time a hard-rock mine was used like a scuba tank to store compressed air.

The dense limestone contains about 2.7 million cubic feet of tunnels and shaftways, some of them thousands of feet long and as deep as a half-mile.

Broome, 68, an engineer who has long been interested in energy projects - he built a hydroelectric generator at the Ontelaunee Dam in Reading and has been involved in several other power projects - perked up five years ago when he heard about the mine's potential.

He and Hoe, 47, who had worked in previous partnerships, formed the CAES Corp. in 1989 - it stands for Compressed Air Energy Storage.

Since then, Broome has pored over underground maps and interviewed the engineers who bored the 18-foot-diameter tunnels for transporting train cars containing iron ore. The tunnels criss-cross the property at different levels.

But it was only recently that they signed the lease with Carr's company, Morgantown Properties. The lease gives them five years to build the plant, and then runs for 29 1/2 years once the plant is operating.

They say the plant would produce 110,000 kilowatts of electricity, enough to supply 66,000 homes.

ENVIRONMENTAL EFFECTS

Broome and Hoe angered environmentalists several years ago by proposing to build a 2,500-kilowatt hydroelectric generator at the Flat Rock Dam on the Schuylkill near Manayunk (the project did not pan out).

They said that the air-compression system should have insignificant environmental consequences.

The machines that pump the air underground would be conventional electrical compressors, they said. And the turbines that power the generators would be the same gas-powered machines that utilities use to produce peak electrical power.

The use of underground chambers for energy storage is not novel. Oil companies already keep petroleum reserves and pressurized natural gas in impervious caverns.

Broome and Hoe have applied for a grant from the Electrical Power Research Institute in Palo Alto, Calif., the research arm of the electric power industry. The institute helped finance Alabama Electric Cooperative's $65 million compressed-air operation in McIntosh, Ala.

"The key issue is cost," said James Birk, a scientist at the research institute. "If you already have the cavern there, you're way ahead of the game."

ENERGY STORAGE

Broome and Hoe plan to seal off the ends of the vertical shafts and tunnels to create a series of subterranean chambers. The concrete plugs would prevent the pressurized air from escaping into porous rock where iron ore was extracted with explosive blasts.

"The real significant unknown is whether this mine will hold air," Hoe said. "We're confident it will."

Each night the plant operators would turn on compressors and pump air into the mine until it reached a pressure of 1,000 pounds per square inch (atmospheric pressure is 14 pounds per square inch).

During the day, the operators would open valves, causing a blast of air to rush out and run the turbines.

As the air rushed out, it would expand rapidly and become very cold. It would need to be heated with natural gas to prevent it from freezing the turbines.

Because the air would already be pressurized, the turbines would consume about a third of the gas that a conventional gas turbine requires.

Still, energy storage systems like the one Broome and Hoe envision do not conserve electricity. In fact, they use 20 percent to 30 percent more energy than they generate.

They are attractive to energy planners because they use electricity at night, when utilities like to keep their generators running because it is more efficient than turning them off. And they produce power during the afternoon hours, when utilities strain their generators to meet peak demand.

"What we're banking on is the differential between peak and off-peak power," Hoe said. "As long as the differential remains, the economics work."

Hoe said that to secure financing, his venture would need to reach a deal with a utility to buy the power.

Two utilities - Pennsylvania Power & Light Co. and Metropolitan Edison Co. Inc. - serve the mine property. Philadelphia Electric's territory is also nearby. The plant could also sell electricity to a more distant utility.

Carr, the mine owner, says the tenants of his industrial park or his Victorian village also might contract to buy the power.

Never short of ideas, Carr also saw potential uses for the seven-acre water reservoir that Hoe and Broome want to build on the surface. The reservoir would store water that would flow into and out of the mine as it was alternately filled with and emptied of compressed air.

"The water would flow in and out of the reservoir like the tide," said Carr, who envisions the water flowing through elaborate channels, fountains and moving sculptures.

"You could sit there and watch the water ebb and flow."