18 years have passed since the 9/11 attacks took place in New York and it had taught several lessons not only to the United States but the whole world. 18 years back on this day, four passenger aircrafts operated by United Airlines and American Airlines were hijacked by 19 al-Qaeda terrorists. Two of these aircrafts - American Airlines Flight 11 and United Airlines Flight 175 were crashed into the North and South towers of the World Trade Centre complex located in Lower Manhattan which caused both the 110 story towers to collapse. The third plane American Airlines Flight 77 was crashed into the headquarters of the U.S. Department of Defence, the Pentagon leading to the partial collapse of the building's west side. The fourth plane United Airlines Flight 93 was headed towards Washington DC, but it crashed into a field near Shanksville, Pennsylvania after its passengers overpowered the hijackers. This series of attacks killed 2,996 people and injured more than 6,000. An estimated $10 billion worth of damage was caused related to infrastructure and property. This attack had showed the world that no country is invulnerable when it comes to terrorist attacks and global security agencies can never lower their guard at any point or oversee any intelligence reports in their fight against terrorism. Several memorials have been built for the victims of the 9/11 attacks including the National September 11 Memorial and Museum in New York City, the Pentagon Memorial in Virginia and the Flight 93 National Memorial in a field in Pennsylvania. A memorial ceremony was held today morning in Lower Manhattan, in respect of the victims of the attacks.