Some energy-conscious people are going underground, and they're taking their houses with them.

Jutting out of south-facing hillsides, underground houses with wide glass windows are built to bask in the sun. Sunk into ground, they hide out from cold northern winds, and nestle into the constant subterranean temperature of 52 degrees. For the faithful of passive-energy techniques, the direction of salvation is downward.

"A lot of these are built by individuals with the adventurous spirit," said Loring Young, a marketing manager for Milliner Construction Inc. of Frederick, which opened its second underground house this this month. "It's not really new. It's just not done that often."

It takes someone willing to pay an additional 20 percent to 30 percent for construction costs. It takes someone willing to sacrifice a view of the backyard (though it remains available if you want to walk around and up to the roof, which happens to be where the backyard begins.) It takes someone who prefers landscaping to house painting.

The most recent of that ilk locally is Mark Clark, a dairy farmer who hired Milliner to build TerraSol, his underground home, on a 300-acre farm outside of Burtonsville, Md. Young said it cost about $200,000 and took five months to build. He said Clark will move in in February.

The finished product is essentially a block of masonry with a climate unto its own, insulated by a wrapping of Styrofoam and coated in waterproofing, set into a hillside looking down on Rte. 108 about 500 yards to the south.

Milliner built its first passive solar underground house four years ago in Columbia. The designs for that one and Clark's are similar, Young said.

A wide, horizontal spread of windows draws in solar heat. A heat pump assists in heating and cooling the structure. The temperature is maintained by massive concrete walls, which are constructed of 12-inch cinder blocks reinforced with steel rods and filled with concrete. The house is "outsulated" from the surrounding earth by Styrofoam, five inches thick on the rooftop and tapering from four to two inches thick on the walls.

The roof and floors are made of precast, prestressed concrete slabs, Young said. A mysterious, 3/16-inch-thick coat of undisclosed waterproofing covers the entire casting.

"I don't want to say it's secret," Young said, "but it's a company priority kind of thing." (He wouldn't disclose whether it was animal, mineral or vegetable, but said, "The Army uses it on underground storage tanks.")

Cathedral-like clerestory windows high up on the fronting wall bring in not only solar heat, but sunlight as well, spreading it down through the rooms below.

"The preconceived idea of the damp feeling of being down in the corner of the basement has been nonexistent," Young said. The air circulation system keeps fresh air moving through, yet is able to transfer 80 percent of the temperature from outgoing to incoming air.

To meet fire codes, each bedroom has a window, including the master bedroom built in the back of Clark's farmhouse. In that design, the bedroom extends out from the side of the house just enough for a window, which also faces the front.