As a large crowd of concert-goers waiting for a reggae show to start milled around at the Gallivan Center in Salt Lake City, Utah last Thursday, suddenly something very strange happened: roughly 100 people dropped to the ground and didn’t move. 

Onlookers could have been forgiven for thinking there was something very wrong until they spotted the huge red banner being unfurled from a nearby parking garage, painted with big white block letters: “CLIMATE CHANGE KILLS”.

For five minutes, the group lay still on the ground, as people around them stared and tried to walk around them. Finally, a whistle sounded again and they got up without a word, disappearing into the crowd.

This “flash mob”, an event in which a large group of people suddenly assemble in a public place and take part in an unusual action for a brief period of time, was organized online to make a point about global climate change. Participants had been waiting for the sound of whistles and a text message that said “Go!” before they hit the ground.

The stunt was slated to start at 8pm, but was delayed because organizers trying to unfurl the 30-foot banner were dodging the parking garage’s security guards.

"I don't think there was anybody who didn't notice," organizer Flora Bernard told the Salt Lake City Tribune. "As long as they're paying attention, that's enough."