Warning: Contains content some may find distressing

As the world, and the world's media watched the billowing smoke, most pondered that it had to be an accident. Sixteen minutes later, all realised that it was not an accident; it was an attack, when at 09:03am, a second plane, United Airlines Flight 175, crashed into the South Tower.

Reports then began to an emerge that another hijacked plane had crashed into the Pentagon while a fourth United Airlines Flight 93 crashed into a field in Pennsylvania.

The September 11th Attacks, or 9/11 as they are now commonly known, were four coordinated suicide terrorist attacks carried out by al-Qaeda and masterminded by Osama Bin Laden.

In total, 2977 innocent people died whereas thousands upon thousands more were injured.

On this September 11th, as we remember all of the victims of this most heinous attack, people have been taking to social media to share footage of 9/11 which went unseen for over twenty years.

The footage, captured by Kevin Westley, was shared to YouTube in the early 2000s, however unbeknown to Kevin, he had set his video to private and so it went completely unwatched for twenty years until he finally set it to be public.

The video, which now has more than six million views, shows the moment the second plane strikes the South Tower, as people standing on the ground look up in horror and confusion.

One person in the video can be heard screaming: "He did that on purpose." Another cries: "What's going on?!" Many others can be heard screaming and crying.

What perhaps sets Kevin's footage apart from the rest is that he captures the human element of the attacks and the raw horror it cast upon all of those watching from down below.

Many people took to the comment section below Kevin's video to voice their opinions on the video.

One wrote: "This could be the most rare historic footage I've ever seen because it captures everyone's raw reaction."

Another wrote: "These raw recordings shouldn’t be censored. This shows the confusion, the terror and above all the uncertainty. I was 9 years old and living in Tennessee on 9/11. To all police, firefighters, and EMTs who did their duty until the very end, God bless you. To the countless others who died on that terrible day, May you rest in peace. You will not be forgotten!"

A third commented: "Thank you for posting this. No matter how long it was since 9/11 we are still taken back to that very moment in history. It will be 23 years in mere weeks and it still sends a chill down my back and tears begin. God Bless America and to those who lost their lives. Forever remembered never forgotten."

And a fourth wrote: "The fact at 2:00 you can hear and see the plane before it even gets close to the building is the most rarest footage ever, I've never seen any footage that can see the plane entering new york and heading to the building. That is terrifying, and the reactions from everyone around sends chills down my bones. Cause at this moment the day was only beginning, from there the day would increase in attacks.. I live in Canada and I hope all of the victims of this day will never be forgotten.."

Our thoughts go out to all of those who are still affected by the horrific events of 9/11 today.