Sentence:  Vowing that he was “never going to forget the lessons of that day,” President Bush paid tribute last night to the victims of the Sept. 11, 2001, terrorist attack, laying wreaths at ground zero, attending a prayer service at St. Paul's Chapel and making a surprise stop at a firehouse and a memorial museum overlooking the vast <WSD>gash</WSD> in the ground where the twin towers once stood. 
Meaning of the word marked with <WSD> tags:  A deep cut.