Their quarter-mile high towers are gone, and now engineers must be careful of the underground regions below the destroyed World Trade Center. The immense basement, comprised of six floors covering 16 acres (so vast that a giant subterranean retaining wall was required to protect it from the nearby Hudson River), is much like a dam around the Trade Center's base. It is three feet thick and made of concrete with steel reinforcement. But if engineers are not careful, they say, parts of the wall could collapse, adding to the chaos caused by terrorists. "Since the wall is no longer braced by the floors that used to brace it," Matthys P. Levy of the engineering firm Weidlinger Associates and author of the book "Why Buildings Fall Down" told ABCNEWS.com, "you cannot remove some of the rubble that's adjacent to the wall without endangering the wall, which means that the wall could actually cave inward." All this seems unlikely, and engineers are making plans to keep it that way. But part of the cause for their concern is that the soil surrounding the Trade Center site is landfill. (The original Hudson River shoreline runs right under the site.) If nothing held the containment wall up from the inside, the soil and the nearby river would naturally try to push it over. One solution, say engineers, is to anchor the wall with hundreds or even thousands of steel cables, drilled deep into the ground. But that would be a giant project and one more thing to slow the recovery down. "We have to do it right," says George Tamaro of Mueser Rutledge Consulting Engineers. "The implications are very serious."