The trip -- which felt more like spelunking through caves or archaeological ruins than touring an urban structure -- began at the opening of a downward-sloping truck ramp on Barclay Street, one block north of the trade center site, where all freight was once delivered to the trade center. Beginning at the site's northern boundary -- at the western edge of the L-shaped 5 World Trade Center, which burned but did not collapse -- the trip was constrained by several elements of the smashed topography created by the collapse of the twin towers.

To the west, a giant hole punched through the middle of the United States Custom House by falling debris from the north tower continues to the bottom of the basement. Steel beams dangle from the edges of the hole like ragged tapestry and form a wildly chaotic pile in the center. To the southwest is packed debris from the north tower itself, and to the south, many basement floors have been crushed by debris hurled from the south tower.

As solemn as it is, the passage below is not just a study in destruction. As respirators dangled from the necks of everyone else in the small group, John O'Connell, a rescue worker with the Fire Department, smoked a big cigar.

''It's my respirator, it's my oxygen indicator and it's my explosion indicator,'' Mr. O'Connell said. ''The only problem, the explosion indicator, it works only once.''

After a walk southward down the truck ramps and a dogleg right, to the west, the dancing flashlights illuminate the edge of the debris that fell nearly straight down through the north tower and collected down here. At first the mind simply refuses to accept what the eyes see -- the recognizable traces of 20 floors, much like geologic strata, over a 10-foot vertical span.

In one place, the steel decks of half a dozen floors protrude like tattered wallpaper, almost touching where they are bent downward at the edge. ''You're looking at roughly 60 feet of the building, smashed into about 3 feet,'' Mr. O'Connell said.

A three-foot stalagmite of steel, which looks for all the world like a drip candle, sits next to one of the immense steel columns that held up the north face of the tower.