REGIONAL REPORT / THE FIRE BELOW : Undergound Blaze Makes Coal Miners Fear for Jobs : Tunnels are sealed and may stay that way for a year. The firm vows to reopen, but some workers seek new careers.

Until it caught fire and began spewing poisonous smoke like a suddenly awakened volcano, the Mathies coal mine was a sleeping subterranean giant.

In fact, most of the thousands of people living in these serene rural suburbs 16 miles south of Pittsburgh never even knew that the massive monster sprawled several hundred feet under their homes, schools and businesses.

The Mathies mine is the size of Manhattan, an incredible 47-square-mile grid of tunnels 90 feet apart and barely miner’s-cap high. For 50 years, it has been quietly chasing part of the famous “Pittsburgh seam,” the seven-foot-thick layer of bituminous coal that lies hundreds of feet beneath much of southwestern Pennsylvania’s rolling hills.

But the Oct. 17 underground fire--an increasingly rare occurrence in the coal industry--may prove fatal to the mine and to the jobs of its 400 employees, three-fourths of whom are coal miners making $600 a week.

Production at the Mathies mine has ceased. Its nine entrances and air shafts have been sealed with steel and concrete caps in an effort to smother the fire, which still burns 300 feet below the mine’s Thomas portal, or entrance, where it started.

“It’s very rare we have a fire this bad,” said Steven Luzik of the federal Mine Safety and Health Administration’s Pittsburgh district office.

Most mine fires are electrical or are caused by friction and are easily controllable, Luzik said. Across the country last year, there were only seven mine fires that required 30 minutes or more to put out.

In 1988, there were 15 fires, including one that closed the Marianna mine in Marianna, Pa., which remains sealed. But most sealed coal mines are eventually reopened, Luzik said. It all depends on the amount of coal reserves in the mine and the cost of resuming operations.

No one knows for sure, but the early morning Mathies fire may have started along the main coal-hauling line when a roof of coal containing the bare high-voltage wire that powers the mine’s 50-ton electric locomotives collapsed.

The fire quickly spread out of control. After a small explosion slightly injured 11 firefighters, the mine was evacuated. Soon, the order came to begin sealing the mine, a five-day job.

Meanwhile, noxious sulfuric smoke poured from the Thomas portal air shaft, the last shaft to be sealed. At night, the thick, sooty cloud settled in the valleys around Thomas like a deadly fog. Carbon monoxide reached unhealthful levels and 100 families had to be evacuated.

Now the smoke is gone and residents have returned. But smothering the fire will take at least six and maybe 12 months. Then the Mathies Coal Co., a subsidiary of National Steel Corp., will inspect the damage and decide whether to reopen its mine.

Although it is one of the country’s largest mines, Mathies’ production is modest--1.6 million tons a year of low-sulphur coal used in making steel and steam, sold at $35 a ton. Nearly 1 billion tons of coal was produced in the United States in 1988 by 1,400 underground and 1,400 surface mines, according to the National Coal Assn., which represents coal mine operators.

U.S. coal production is up since 1984, but employment in the industry has fallen from 177,000 in 1984 to 135,000 in 1988. About 900 mines are in Kentucky and 600 are in West Virginia, the country’s No. 1 coal-producing state.

Mathies is only one of Pennsylvania’s 432 active mines, but it still holds between 35 million and 50 million tons of reserves, and Mathies’ president, Vaughn Knapp, vows to reopen the mine.

Nevertheless, miners like Clark Hayes, 35, are already planning new careers. Hayes, as hulking and strong as a pro football tight end, has been a “roamer,” or general laborer, at the Mathies mine since he was 19.

He thinks the mine may never reopen, or that it might reopen only partly. He’s lucky, he said, because he has some real estate he can fall back on.

But leaving coal mining will be hard, he said. When his boss asked if he wanted to help seal up the pit mouth, Hayes said he told him: “No. I’m not going to bury . . . my best friend.”

Even harder will be leaving his fellow miners, who are as close as family.

“We work in an occupation that’s not the safest in the world,” he said, standing in a mine entrance parking lot giving goodby bearhugs to friends he might never see again.

“You have a lot of people who are constantly watching over you and you’re watching over them. You’re concerned about one another. It’s a very emotional occupation.”