Some of the heroes of Sept. 11, 2001, performed their lifesaving work at the World Trade Center many years before the attack.

Arturo Lamberto Ressi di Cervia, who died last month at 72, supervised the construction of the slurry wall around the trade center foundation in the 1960s. When the towers collapsed in 2001, and the wall began to strain under almost unthinkable pressures from the surrounding water table, his work paid off.

The wall held.

Because the slurry wall held, the 70-foot-deep foundation did not fill with groundwater. And because of that, the PATH tubes were not submerged. And because of that, the subway tunnels below the PATH tubes were not inundated.

How much worse could Sept. 11 have been? Imagine if Hurricane Sandy had followed the terrorist attack by a few hours.