Why do we memorialize Sept. 11?

As you descend into the immense subterranean spaces and through the winding, trauma-saturated displays of the National September 11 Memorial Museum, that question will hardly seem as urgent as it does when you leave, feeling peculiarly uncertain about the answer. But at first, there is nothing to ask: You are submerged in an eerie underworld, haunted by ghostly memories and the murmur of gibbering voices.

Ordinary artifacts are relics of ruin and death here: a Metro Card used by one victim, a lacrosse stick owned by another, shards of World Trade Center window glass, a tattered seatbelt from Flight 77 torn off its mooring. The sounds are television broadcasts and taped messages, secret phone calls from hijacked planes and rescuers on radios wrestling with apocalypse. The voices are those of the unknowing, thinking themselves spared as the first tower is hit; and those of the doomed, knowing they had just seconds for farewells. No wonder the galleries include stands with tissue dispensers.

The power of these exhibits, which chronologically conduct visitors through the day, is unsurpassed in contemporary history museums. And in darkened screening rooms, eyewitness accounts are heard, while locations are pointed out on schematic video sketches of the towers, the Pentagon, or United Airlines Flight 93, the multiple voices masterfully woven into seamless narratives.