Packed into narrow alleyways lined with bars and clubs, thousands of people travelled to Itaewon, an area of Seoul, for the city's first Halloween festival since the COVID-19 pandemic.
But as the crowds grew throughout the night to unmanageable levels, the celebration turned disastrous as a crush in one alleyway left at least 154 dead.
Sky News has analysed videos from social media to show how it took just 30 minutes for the situation to spiral out of control.
They also confirm an apparent lack of police or security presence in the build-up to the crush.
The revellers turning up for the celebrations were predominantly young, with many travelling to the area via Itaewon train station.
Some livestreamed videos of their evening on Facebook. The screenshot below, taken from one of these livestreams, shows crowds beginning to form on the main road by the station at 8.10pm.
The main road is busy, but cars are still able to drive freely and there is still space between people.
Over the following hours, the crowds grow in size. They funnel into a series of small side streets behind the main road around the Hamilton Hotel, highlighted in yellow on the map below. These streets are small and those running north to south slope steeply.
At 10.06pm, people can be seen in the main alleyway running east to west still freely moving but slowly. Some people are holding each other to avoid being split up by the crowd as they pass an Irish bar in the background.
Later, 150 metres east of where the footage above was shot, a denser crowd grew on one of the steep and narrow side alleys. People were so close to each other that any small movement rippled through the crowd.
In the footage below pop music is heard blasting out amongst cries of anguish as the packed-in crowd sense danger.
People in this street narrowly avoided disaster. It would be another steep alleyway on the other side of the Hamilton Hotel where the deadly crush would occur, highlighted in dark yellow below.
This short video shows this alleyway west of the hotel earlier in the evening. The crowd is spread wall to wall across the narrow passageway - which sits on a 20% gradient - with no escape routes.
Around the corner, Janelle Story, an eyewitness who spoke to Sky News, filmed her view as the crowd surged. This video was captured at 10.34pm.
Story told Sky News: "Very suddenly this wave of people just came rushing towards us with this incredible force and urgency, and in the moment I stopped filming because it got really serious and scary."
She was 40 metres from the western alleyway by the Hamilton Hotel where a crush was occurring. By matching the building exterior with existing imagery from Google Streetview, Sky News confirmed the footage was taken outside the Irish bar seen in the earlier video at 10.06pm.
In less than half an hour, a busy but free-flowing crowd had turned dangerous.
By 10.45pm it became clear something was very wrong. Livestreams from the main road show ambulances and emergency personnel heading towards the Hamilton Hotel.
In the surrounding crowds it also began to dawn on people a serious incident had occurred. One young man had been live-streaming the party on the street with his friends having fun. As they moved towards the Hamilton Hotel, the mood changed.
At 11.15pm he filmed the crowd parting the way as paramedics moved through with patients on stretchers.
The video below shows the scene that the emergency services were heading from. A surge had forced the crowd to fall over, likely made worse by the steepness of the alleyway.
Emergency workers can be seen trying to free those caught in the crush. People try climbing the walls in an attempt to escape, whilst those on a small set of stairs leading to a side door into the hotel try to lift people to safety.
Reports suggested some people were trapped for over an hour. Other videos posted online show paramedics and members of the public attempting CPR in the moments after the surge. These videos are too distressing to share.
In the immediate build-up to the crush, few police officers or security personnel can be seen in the busy alleys on the livestreams analysed by Sky News. Questions are now being asked of the authorities' management of the situation.
Graphics: Arianne Cantwell
The Data and Forensics team is a multi-skilled unit dedicated to providing transparent journalism from Sky News. We gather, analyse and visualise data to tell data-driven stories. We combine traditional reporting skills with advanced analysis of satellite images, social media and other open source information. Through multimedia storytelling we aim to better explain the world while also showing how our journalism is done.