Paramedics rushed 25 people to hospital for carbon monoxide poisoning after they attended an illegal “cave rave”.
The party had been planned about three weeks in advance, according to the 24-year-old organiser who shared the secret location on Facebook, in Oslo, Norway.
The organiser said they never meant to have so many people there after a whopping 200 descended on the rave and it “got out of control”.
Venue owner Vidar Haukeland, CEO of the Foundation Diakonissethusset Lovisenberg, said the cave had previously been sealed up but that it was opened up before the event in what was described as a “serious burglary”.
Officers found the venue just before 4am on Sunday morning after a patrol came across several dazed people walking the streets.
Oslo police inspector Emil Lorch-Falch told media there was just one square metre hatch which was the only way into the 500square metre bunker which was buried at about 70 metres underground.
Deputy chief of staff for the Oslo Fire and Rescue service, Lars Magne Hovtun, said a danger of suffocation, lack of fire escapes and poisoning were just some of the hazards associated with the event.
He added: “We would have stopped this party immediately even if the airflow had been good.
“The risk of suffocation was just one of many things posing a life-threatening situation.”
Carbon monoxide, a deadly-odourless gas, can stop a person’s blood from carrying oxygen – potentially killing them.
Mr Hovtun claimed the carbon monoxide level in parts of the bunker was “many times” more than the lethal level when paramedics arrived.
It was estimated that the oxygen level was about 16% compared with a minimum safe concentration of 19%.
He added: “We fully expected to find several dozen unconscious people inside.
“We have to say that based on risk potential, we are quite surprised that this did not all turn out much worse.”
Two officers were among those being treated for poisoning and that all the revellers are reported to be out of critical condition.