This year marks the 21st anniversary since the loss of almost 3000 lives in the 9/11 attacks.
The morning of 11 September 2001 started like every other working day at the World Trade Center - as 19 hijackers boarded commercial planes headed for different locations in the US.
Four planes were hijacked by Islamic extremist group al Qaeda, with one hitting the Pentagon just outside Washington DC, and another crashing into a field in Pennsylvania.
However the moment etched in the memories of the millions of people who watched the horror story unfold live on television that day, was the collapse of the Twin Towers in New York.
Although conspiracy theories have emerged - and have been debunked - around why such sturdy structures fell to the ground, experts have concluded why exactly the World Trade Center collapsed on that fateful day.
Here’s what you need to know about the famous Twin Towers - and what stands in their place now.
Why did the Twin Towers collapse?
On the morning of 11 September 2001, the World Trade Center buildings in New York were struck by two hijacked commercial planes.
The first plane hit the North Tower at 8:46am, and the second struck the South Tower shortly afterwards, at 9:03am.
In under two hours, both 110-story towers had collapsed.
The Twin Towers made up two of the seven World Trade Center buildings in the financial district of Lower Manhattan.
The towers were the centerpieces of the complex and upon completion were then the tallest buildings in the world.
The debris severely damaged or destroyed more than a dozen other nearby structures, ultimately leading to the collapse of 7 World Trade Center at 5.21pm.
In September 2005, the National Institute of Standards and Technology (NIST) published the results of its investigation into the collapse of the buildings four years after the attack.
Investigators said World Trade Centre 1 and World Trade Centre 2 failures (the Twin Towers) were “brought on by multiple factors”.
Those included structural damage caused by the aircraft impact, extensive dislodgement of the sprayed fire-resistive materials or fireproofing in the impacted region, and a weakening of the steel structures created by the fires.
Meanwhile, the collapse of World Trade Center 7 was caused by a single initiating event - “the failure of a northeast building column brought on by fire-induced damage to the adjacent flooring system and connections.”
The towers were eventually replaced by the One World Trade Center, which began construction in 2006 before opening in 3 November 2014.
How many floors were there in the Twin Towers?
Each of the twin towers had 110 floors, 97 passenger lifts, and 21,800 windows.
1 World Trade Centre (the north tower) and 2 World Trade Centre (the south tower) provided nearly 10 million square feet of office space for about 35,000 people and 430 companies.
The pair were designed by architect Minoru Yamasaki and officially opened in 1972, four years before they were finally completed and became iconic features of the Manhattan skyline in 1976.
How many people died in the Twin Towers attacks?
A total of 2,996 people were killed in the 9/11 attacks, including the 19 hijackers that took control of the planes.
The majority of the fatalities occurred in New York, and more than 6,000 additional people were injured.
All passengers and crew of the four planes involved in the attack were killed, and a further 2,606 died at the Twin Towers, either at the scene or later on from their injuries.
At the Pentagon, 125 people also lost their lives, most of whom worked for the United States Army or the United States Navy.
A total of 40 passengers and crew died on United Airlines Flight 93, which crashed into a field in Pennsylvania. It’s thought that the Capitol building or the White House was its target.
The FBI states that more than 400 first responders, including 60 law enforcement officers, were killed.
The youngest victim of the attack was Christine Lee Hanson, who was only two years old. She died on Flight 175, which crashed into the south tower of the World Trade Center, along with her parents Peter and Sue.
The eldest was 82-year-old Robert Norton, who was on Flight 77, which crashed into the Pentagon, with his wife Jacqueline. They were travelling to a wedding.
Countries around the world lost citizens in the attack, including the UK with 67 deaths, Greece with 39 deaths, Australia with 11 deaths and South Korea with 28 deaths.