Impact of plane hitting tower during 9/11 'heard clearly for the first time' on 25th anniversary of tragedy

25 years two planes struck the World Trade Center as part of a terrorist attack which hijacked four planes in the US and targeted them at major American landmarks.

The hijacked flights hit the Twin Towers and the Pentagon, while a passenger revolt on the fourth aircraft brought it crashing down in a field in Pennsylvania.

On the anniversary of the attacks people have been sharing some of the most vivid memories of the day, as well as footage and evidence not previously seen.

The documentary 9/11 The Unseen Footage has shown some of these along with witness testimonials to paint the clearest picture of what happened.

The visuals are striking, and others have shared the audio of the second plane hitting the World Trade Center on 9/11 as they were taken aback at what the impact sounded like.

Footage of the already struck North Tower captured the moment a plane hit its counterpart, including the sound of the plane's impact before the camera panned over to see the developing fireball from the crash.

Plenty of those reacting to the footage said they'd never actually heard what it sounded like, with others saying they'd 'never thought about it before'.

Others sympathised with the people who'd been in New York City on 11 September, saying they couldn't 'fathom what it was like being there' to actually witness it in person.

For those who were there it was not initially clear whether or not the first crash was a freak accident or intentional, but then the second plane struck the South Tower.

Some of those who were on the ground recounted their experiences, including Dr Gordon Huie.

He explained that when he saw the attacks he first started heading towards the towers, but turned around when the North Tower collapsed as he realised there was nothing he could do.

Changing direction, he headed for the nearby Beekman Hospital where he was put to work as a first responder sewing people up on a conference room table.

It was only when there were no more patients coming through that he got the call from his father to say his sister had been to the World Trade Center for a meeting at the Windows on the World restaurant, nobody who had been there on the day of the terrorist attacks survived.

Another witness to the catastrophic attacks was bond trader Stefan Horlacher, who had been working two blocks away from the Twin Towers and said he felt the 'vibration' from the first plane strike.

He went outside and witnessed people jumping to their deaths from the tower and then saw the second plane strike, showering the streets with debris.

A woman standing next to him, Debbie St John, was struck by debris and cut down her back, Stefan tied his shirt around her torso and along with others protected her from being trampled by people running away.

Debbie was the first hospital patient admitted on 9/11 and the last to leave, as she spent 18 months undergoing 50 surgeries before she was well enough to leave.