A dust cloud engulfed Kayla Bergeron and Patty Clark on the debris-scattered plaza as the World Trade Center's south tower collapsed on Sept. 11, 2001.

Having just descended 68 and 65 floors, respectively, the two Port Authority employees lingered near the base of the north tower before remembering that a narrow, two-story staircase led down to the street.

Bergeron and Clark picked their way across fractured concrete and shattered glass to the stairs, made their way to Vesey Street and walked away.

Minutes later, the north tower collapsed, destroying everything in its footprint, but leaving the staircase.

On Wednesday, one of the nation's largest preservation groups began a campaign to save the staircase -- the only above-ground remnant of the Twin Towers -- from one thing that can bring it down: the wrecking ball.

The staircase sits where one of five new towers would be built by Silverstein Properties, and the Port Authority of New York and New Jersey plans to excavate the site so it can build an underground wall to protect the building's foundation from groundwater.

As a result, the National Trust for Historic Preservation named the staircase one of the most endangered historic sites in the United States.

"We need this staircase as a tangible link to that day, as a symbol of America's strength, and determination to survive," Richard Moe, president of the organization, said at a news conference in Washington, D.C.

Silverstein executives said Wednesday they would work with the Port Authority and preservation groups to preserve the staircase, possibly moving it to another site or housing it in a planned memorial or museum.

A Port Authority spokesman said the agency was committed to preserving the staircase and is exploring all options.

Dubbed the "Survivors' Staircase," Moe and others said the stairs were used by scores of people like Bergeron and Clark to escape the disaster.

Preservationists said they prefer that the staircase remain where it is because of its historical value. But some concede that a museum or memorial may be the best spot.

"I think it would be appropriate as long as it's close by," said Richard Zimbler, a member of the World Trade Center Survivors' Network, a group of about 500 that has written letters petitioning the National Register of Historic Places to designate the staircase.

"I live five blocks away from the staircase, so I see it every day from my living room," Zimbler said "It's an element of history, and we want it there to help educate everyone who comes to the site."

Amid the much-publicized debate about the World Trade Center memorial, the staircase has received scant attention. Indeed, the thousands of tourists who lined Church Street on Wednesday to take photos of Ground Zero mostly ignored it.

The staircase is a large, but inconspicuous piece of concrete flanked by a trailer and a large Dumpster. Most of the 23 steps on the first flight are broken and barely usable. But the second flight, which led to the long-gone plaza, is in perfect condition.

Construction workers continually pass it on their way to finish the Port Authority's transportation hub below the site and continue work on the Freedom Tower's foundation.

Moe said the stairs have been weakened by the constant vibrations from the construction and should be stabilized quickly.

The Lower Manhattan Development Corp. has begun the long process of getting federal historic designation for parts of the World Trade Center remains, preserving them for posterity.

Bergeron and Clark support the effort. Both say the staircase helped them and scores more escape death.

Clark remembered a man telling the two shortly after the south tower fell to remain under the eaves of the north tower to avoid being hit by falling debris.

"Everything was in gray ash, but we knew the staircase was there," she said Wednesday. "We knew it was the only way out of the plaza."

Washington Correspondent Herb Jackson contributed to this article. E-mail: fallon@northjersey.com.