"I've arrested people for this a number of times," he said. "It's kind of a college kid gathering place. They make gas bombs and drop them down the mine shaft. A bomb hits the wall of the mine shaft and sends a huge fire ball up 200 feet in the air. ... It's pretty spectacular, but it's incredibly, incredibly dangerous."
On Saturday, as the students dropped small bombs and fireworks into the shaft, a third group arrived with "large quantities of gasoline," Cannon said.
"Someone decides he wants to go a bit bigger, so they put two or three gallons of gasoline in a jug or cooler and put newspaper in it as a makeshift wick," Cannon said.
Several of the spectators were sitting on the grate with their legs dangling through the spaces when the jug of gasoline was knocked over.
"There were huge burning flashes and fireballs," Cannon said. "Two or three series of them came up. Then the wall of the mine shaft caught on fire."
Several of the spectators panicked as they tried to free their legs from the rebar grate, Cannon said.
"It was a chaotic situation for those kids," he said.
The group tried to call for help, but the site is out of range of most mobile providers, Cannon said. Some of the spectators and victims drove themselves to hospitals. Seven of the victims suffered serious injuries and were later transferred to the University Hospital burn unit, Cannon said.
Officers are investigating possible felony charges against those who made the gasoline bombs in addition to trespassing charges.
"People say that's pretty heavy-handed, but ... we have to do this so they won't go out there and get burned or killed," Cannon said. Cannon said he has seen groups of students from most of the state's colleges and universities playing with fire in the mines in recent years.