Because Volunteers didn’t know how many people were trapped alive in the wreckage, firefighters and other rescue workers had to search carefully through the unstable piles of rubble for air pockets, called “voids,” where they might find people who had been unable to escape from the collapsing buildings. Almost as soon as the World Trade Center’s Twin Towers fell on September 11, 2001, thousands of firefighters, police officers, construction workers, search-and-rescue dogs and volunteers headed to Ground Zero to look for survivors . Where else is Ground Zero concept being used ?

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