Anyone who was watching television on September 11, 2001 will remember many things... the horror of the second plane hitting the World Trade Centre, then the first tower falling, then the second.
But a lot of what we remember will be linked to the sounds we heard. The roar of the massive skyscraper coming down, or the eyewitnesses sharing what they saw, shaking and in shock.
There were so many sounds that day which reflected the gravity of the attack, but none captured the drama playing out second by second as much as the radio transmission broadcasts from both air traffic control and the Fire Department of New York.
As part of the lengthy investigation into the attacks, these audio files were released in full. Due to my interest in how this event unfolded and how it was covered by the world's media, I decided I'd listen to every second of every recording.
These tapes aren't easy to listen to and please be cautious about listening to them, but they tell so many different stories about what happened on that one day.
You hear a call being taken from Betty Ong, a flight attendant on American Airlines flight 11. Ong is considered to be the first person to get word out that the attack was taking place.
As you hear in the video above, she calls American Airlines' reservation centre and relays information to authorities not only about what was happening, but important details such as where the terrorists were seated on the plane. This led to easy identification of the suspects and gave investigators something to work with even as the attack was in its very beginning stages.
You also hear the moment one air traffic controller realised just how serious the incident was as they're told the attackers had made their way into the cockpit. He could only manage two words.
While everyone will remember the planes flying into the World Trade Centre, that was only part of it. There were four aircraft hijacked that morning - the two headed to New York that killed thousands, while another flew into the Pentagon.
United 93 was heading for Washington, DC, home of the US Capitol Building and the White House. But passengers and crew are thought to have overpowered the terrorists and the plane crashed in Shanksville, Pennsylvania.
Many of the tones of the voices captured on the recordings are of disbelief, others of determination, such as the FDNY crews who ran towards danger knowing that they may never make it home alive.
Hundreds of them didn't.
But the video ends with someone who sounds more calm than most. This person, as yet unidentified, wouldn't have known at the time how historic the words he was about to say were, and how they would be spoken through millions of mouths for decades to come.
"September 11, 2001," he said to a colleague.
And then the recording falls silent.