The unnamed 20-year-old woman told NRK that “everything was great fun for several hours”, but then suddenly the lights went out and the music stopped. “Someone had broken into the room where the generators were,” she said.
“As we entered, the gas hit us in the face. We saw three boys there unconscious. We got very dizzy. It was very scary, we had to keep running out because we were fainting ourselves, but we had to try and pull them out little by little because we didn’t know how long they had been there.”
One of the event’s organisers, who was also hospitalised, told the broadcaster that the generators had been placed in a room with an air shaft and a metal door that had been closed and sealed off with bin liners and duct tape.
“It’s easy to be wise afterwards,” he said. “I don’t regret the event itself, but we could have had someone look after the door: if it hadn’t been opened, the event would technically have been a success. You can look at it as irresponsible, but right there and then it seemed like a good idea.”
I'm not normally someone to cry 'Darwin awards', but there are exceptions ...
https://www.theguardian.com/world/2020/aug/31/oslo-bunker-rave-puts-25-in-hospital-with-carbon-monoxide-poisoning