Mr. Mowatt, 21, worked at Forte Food Service, which served the World Trade Center, and he had been thinking about going to college.

For now, there is nowhere else at ground zero where time is more palpably suspended than in the tubes and tunnels and truck bays that once served the World Trade Center and, before that, the Hudson & Manhattan Railroad, predecessor to PATH.

What purpose did the slurry wall serve the World Trade Center, and what is its significance now?