New York Stock Exchange

When the attacks happened, and because of how close the World Trade Centers were located to Wall Street, trading wasn’t even started. Everyone that had shown up to work that day was told to stay inside until it was safe. Many people inside the exchange reported feeling the ground shake as the two towers collapsed, and the exchange became a refuge for those fleeing the giant cloud of dust, smoke and debris that appeared once the towers fell.
The buildings that hold the New York Stock Exchange were not damaged during the attacks, but a major telephone bunker than held the phone system for the entire area located near the World Trade Center was severely damaged, hence making communication on the floor of the exchange impossible.