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Why it matters:  On a Tuesday morning in 2001, four commercial airplanes were hijacked in an attack that would cost more than 3,000 Americans their lives. The brave actions of 40 passengers and crew aboard Flight 93 thwarted an attack on the U.S. Capitol.

Don't miss: Take a quiet stroll through the 40 Memorial Groves, one in honor of each passenger aboard Flight 93.