In the first approaches to the steam tunnels, the temperature gets up to about 120 degrees, but there are places along the underground passageways where it gets even hotter. It is beyond this point, beyond the baking gloom, where the hobos live.

The big steam pipes run deep underground, from the Consolidated Edison generating plants along the East River in Manhattan. The steam is a byproduct of electricity, and Con Ed is proud, in an energy conscious age, to be able to say that the steam is used twice -- first to turn the turbines that generate electricity and then to provide heat to several Manhattan buildings.

The pipes run out in several directions, like the spines of a fan. Most of them are buried, sealed in concrete and passing under the city along with a maze of other pipes and ducts and tubes and conduits.

But this stretch of tunnels is passable and can be entered from several little-known doorways off Park Avenue or even, to the determined explorer, off certain passenger platforms that line some of the 13 miles of railroad track coiling beneath and around Grand Central Station.

The hobos have known about them for years.

The tunnels run below the lowest level of tracks entering Grand Central. Illuminated by dim, bare bulbs, the passageways have taken on the baked colors of the desert -- ochre, beige, pale brown. On one side, from floor to ceiling, run the networks of steam pipes, the largest ones three feet in diameter, here and there spaced with gleaming piston-like expansion joints to accomodate the fluctuation in temperature.

Everything there is a thick gray dust, part fine-ground steel from the rails far overhead, part grounds from the brake shoes. At some points, the dust has turned to muck from water that seeps through cracks in the walls and leaks in the steam valves.

Cockroaches and water bugs, fat and brown and long as a man's thumb, nest in clusters.

There is endless noise, a hiss of steam, a clanking of machinery, and overhead, like thunder, the trains move in and out of the busiest station in the nation.

There are signs of habitation. Scraps of paper and food are picked over by the insects. A sign painted on the wall says "Waldorf Hotel."

And indeed, this is the sub-basement of the Waldorf Astoria, whose canopy on Park Avenue is six stories above.

It is here in the tunnels, late at night, that the hobos can be found.

A man named Michael Stone lifts his head from a cardboard mattress, and, thinking his visitors are his regular companions, asks, "Do you have a match?"

Stone says he is 62, claims to have spent 25 years in a mental hospital and passed the last dozen years wandering -- homeless, he makes clear, out of choice.

While he talks, tugging on a fierce tangle of black beard, a woman materializes.

She says her name is Mary, and she is looking for her boyfriend. Mary is small, wild-eyed, smelling of wine and unsteady on her feet.

Mary is not very coherant. She is not sure how long she has been living in the tunnels. She points only to the bed of newspapers she shares with her boyfriend and says, "I'm going to die down here."

At some distance from this chamber is another settlement, one considerably more orderly. An earlier exploration had revealed a neat bed of unfolded pages of The Wall Street Journal. The pages formed a mattress an inch thick. The previous day's edition was folded precisely in quarters and rested at the head of the bed like a pillow. Three empty bottles of soda water were set in perfect alignment nearby, along with the current issue of Time magazine.

Now, on second look, the arrangement is roughly the same. And down the passageway, a young man presses himself flat behind a pillar, hoping not to be seen. A flashlight reveals him, and he stares back into the beam.

He is barefoot and wears only a pair of trousers, the cuffs of which come to the middle of his shins. He folds his arms across his chest, unsettled at the unexpected encounter:

He says his name is Jim Hayes, that he is 37 years old.

"How long have you been down here?"

"Couple of weeks."

"How did you find out about the place?"

"Down on 34th Street."

"The Salvation Army?"

"Yeah."

"How long are you going to stay here?"

"I don't know. Not long. I been thinking about going out to Long Island. I been thinking about going to the Sally out in Hempstead for the holiday." ("Sally," in the lexicon of the streets, means Salvation Army.)

"Are you one of these guys who lives like a hobo and owns half a million dollars worth of blue-chip stock?" he is asked.

"Do you think I'd be living down here if I did?"

"Why do you live here?"

"It's warm," he says, grinning. He pauses, knowing that is not the answer being sought.

"I don't know," he says. "It's not so bad. Nobody bothers you. Most of the railroad men, they know who you are. They don't give you any trouble. It's not that scary after you've been here a while.

"The only bad thing is the rats. You get some giant rats down here. But if you stay in the light, they don't bother you. I put a bright light in here, you see. That way they stay away from me. They're really more afraid than you are."

On another night, in the quarters occupied by Michael Stone and Mary, a small party is in progress.

Mary's friend, a large, dark-haired and taciturn man who calls himself Gordon Peter Paul, has turned up. Mary is calm, drinking a cup of eggnog from a carton resting on a dusty steal beam. Stone is reclining on his cardboard mattress, exactly as before.

Mary and Paul are talking to a newcomer -- except that he is not really a newcomer. His name is Al Gahrman, or "Big Al," as he says he is known. Gahrman has come with three bottles of beer, and the eggnog, and has brought with him a mounted photograph of a stretch of railroad track that looks as if it might have been taken in South Dakota.

"No sir," Gahrman said. "That's Long Island, not far from Massapequa. Nice, ain't it?"

Gahrman says he is "just visiting." He says he has been coming down in "the hole" to sleep for "20 years, maybe longer."

Gahrman is clean and neatly dressed. He says he is 50 years old. He does not look like a man who spends his days wandering around in the gloom of the steam tunnels.

But he likes to come down and visit with the hobos. He brings beer and sits for hours telling stories of other places where he has slept and ridden the rails.

"Whenever my girlfriend can't find me," he said, "she knows just where to look. She told me, 'My God, Al there's a whole world down here that nobody knows about.' She's right. There's a whole world down here. Do you know you can go underground all the way up to 125th Street?

There were hobos sleeping in the steam tunnels before Gahrman discovered them -- possibly since 1913, when Grand Central Station was opened. Most of the hobos find out about the tunnels the way Gahrman did -- from other hobos. Gahrman has passed the favor on to other urban and railyard wanderers, who, if possible, like to avoid operations such as the Salvation Army because they value, above all else, independence and solitude. And having to stand in line, for a bunk ticket or a bowl of soup, is a violation of those values.

In that sense, the steam-tunnel dwellers are less like derelicts than the railroad hobos of 50 years ago.

Periodically, according to Noah Caplin, a spokesman for the Metropolitan Transportation Authority and an expert on the arcane lore of Grand Central, transit police make a sweep of the tunnels and drive out anyone they find there. But the hobos, of course, know the area better than the police and drift back whenever they like. Railroad workers tend to dislike the hobos but regard them as harmless.

"It's nice down here, don't you think?" Gahrman continues. "See the sign down there? Waldorf Hotel? Well, what do you think about staying at the Waldorf free of charge? Pretty good, huh? A guy would do a lot worse, right Mary?"

"Right," says Mary, sipping delicately at her eggnog.

"Take old Mike over there," Gahrman goes on. "I used to see him downtown, sittin' in a doorway, and I would talk to him and he was always complaining about his feet bein' cold. So I said to him, 'Man, ain't you heard about the hole?"

"He says he's heard about it but he don't know how to find it. So I take him down here and show it to him. He can't believe it. He loves it. As far as I can tell, he ain't got up since the day he arrived. Right Mike?"

"Right," says Mike, who is lying on his cardboard, gazing into the darkness above him, his hands folded peacefully across his stomach.