The 9/11 terror attacks are forever etched in the collective memory of the world. On September 11, 2001, the world witnessed horrors hitherto unseen as the Twin Towers in New York City, USA, came crumbling down after two commercial flights crashed into them. Another jetliner hit the Pentagon, and a fourth crashed in Pennsylvania.Also Read: Our coverage of 20th years of 9/11 attacksThe moment and its aftermath, chronicled by The Hindu from the ground, completely changed the way the world looked at terrorism. Its scars are still visible 25 years later, in the form of the 9/11 Memorial and Museum and the accounts of many who lost loved ones.What happened on 9/11?At its core, the 9/11 terror attacks involved four planes that were hijacked by terrorists. Their intended targets were the North and South Towers of the World Trade Center in New York and the Pentagon. The target of the fourth flight has been speculated widely, with some saying that even the White House could have been the final destination of that fateful aircraft.From 2021: Worldview with Suhasini Haidar | 20 years after 9/11, what have we learned?The four flights that were involved were American Airlines Flight 11, United Airlines Flight 175, American Airlines Flight 77, and United Airlines Flight 93. The world watched in horror as the city descended into chaos and smoke spread across New York with people jumping from the Twin Towers.