Across the street from the yawning void left when the World Trade Center collapsed on Sept. 11, 2001, sits a center imbued with the memory of the 2,750 people who died there.

The completion of a permanent memorial is at least two years away, and the Tribute Center is the sole space near Ground Zero where a visitor can hear the anxious radio transmissions of a firefighter inside the south tower; see a twisted steel beam from the Trade Center's core; study the ID card of a husband who never made it home.

It has become a destination in its own right, and on Monday, less than a year and a half after it opened, the center welcomed its 400,000th visitor, a man from Kent, England, and his wife.