WTAF?

What are people's guesses?

I'm thinking a high concentration of sewer gas (hydrogen sulfide) -- one sniff and your nose stops smelling it, rapid knock-down and death within a few breaths before anyone realizes they need to get out. Maybe it was accumulating on an upper level and flowed down into the club area?

https://www.theguardian.com/world/2022/jun/26/south-africa-nightclub-deaths-east-london