For the uninitiated, the Atlantic Avenue-Pacific Street subway station in Brooklyn has long been a daunting place, a jungle of confusing signs, dank corridors and complicated twists and turns.

A detailed map helps only slightly, revealing a tangle of 10 subway lines running through the terminal, as well as the Long Island Rail Road.

Making matters worse, over the last four years New York City Transit has been gutting the station, the fourth busiest in the system, and putting it back together piece by piece to relieve congestion underground.

The main problem was a narrow, nearly century-old pedestrian passageway under the tracks, choked with people. Engineers wanted to double the artery's width, a task complicated by the 131,000 people a day who still needed to use it and the nonstop parade of trains overhead.

''The concourses run underneath the street, underneath the 4, 5 and 2, 3 tracks and underneath the L.I.R.R. tracks,'' said Gary Negrycz, a project executive for the Turner Construction Company, which oversaw the work. ''You've got to support all of that, mine out the earth underneath, construct the walls, new roofs, all while the trains run.''

The resulting four-year nightmare of jackhammers, blocked stairwells and rerouted exits will finally end next month, and commuters will get their station back. But most will have missed the feat of engineering that unfolded all around them every day.

The traditional approach to widening the concourse would have been to simply excavate along the edges of the passageway, bolstering the roof in sections. But this was considered risky and laborious. If any of the earth slipped, even an inch, the tracks could dip and force transit officials to close the line above for repairs.

As engineers cast about for ideas, an unorthodox solution emerged: suspend the subway tunnel from its roof.

''By hanging it, the whole structure is supported before you ever take out one shovelful of dirt,'' Mr. Negrycz said.

At first, transit officials were skeptical. The technique was suggested by Irwin S. Toporoff, 70, an engineering consultant and tunneling expert.

''This had never been done before,'' Mr. Negrycz said.

Mysore Nagaraja, president of the Metropolitan Transportation Authority's capital construction corporation, said, ''It was quite a radical scheme.''

But Mr. Toporoff was convinced it would work. Although a subway tunnel is encased in concrete, its frame is essentially a steel box. Instead of propping it up from the bottom, Mr. Toporoff reasoned, the agency could send columns through the tunnel to bear the weight of Flatbush Avenue, while suspending the tunnel from above.

''Anytime you tunnel, you risk losing ground, which can cause settlement'' of the earth, he said in an interview. ''With this, you eliminate the risk.''

Transit agency engineers approved the idea. In July 2000, two gaping access holes into the complex were opened, one on Fourth Avenue and the other through the floor of an old station control house on a traffic island a few hundred feet away. Engineers negotiated with the city's Department of Transportation to close two lanes of traffic on Fourth Avenue for 18 months to give them room to work.

The control house, boarded up for years and defaced with graffiti, was in many ways emblematic of the station's decline. Designed in 1908 by the same architects who designed the Cathedral Church of St. John the Divine, it served for decades as an elaborate, ornamental subway entrance. Thirty years ago, the entrance closed and the control house was converted into a newsstand. In 1980, along with several other ornamental entrances, it was designated a historic site. But in recent years it fell into disrepair.

Schiavone Construction, which did most of the work on the project, hired a company to pick up the control house and move it to a nearby parking lot on the back of a flatbed truck in the middle of the night in early 2000. Through the hole left behind, along with the other one on Fourth Avenue, workers began excacavating and taking in equipment.

Later that year, workers took steel pipes, 10 inches in diameter and filled with concrete and with a steel rod threaded through the middle, and began sinking them from the subway platform into the earth below. Ultimately, the pipes were drilled 50 feet below the platforms and extended to the ceiling of the subway tunnel.

Later, 36-foot-long crossbeams, weighing 300 pounds a foot and hauled into the station by work trains that started in the Bronx, were snaked across the tunnel, just underneath the roof. Most of the tunnel's weight would hang from the crossbeams, including the platforms, stairways and people. The weight of Flatbush Avenue would also rest on the crossbeams and the columns.

Because the roof of the concourse had to be gutted to widen the passageway, the subway tracks themselves would have been left dangling, so smaller columns and supports were placed below them to help hold them up. Most of the weight of the tunnel, however, would still hang from above.