Tommy Alford, who works in a convenience store about three miles from the plant, told CNN that several volunteer firefighters were at the store when they spotted smoke.

CNN: Explosion hits fertilizer plant north of Waco, Texas

The mayor, Mr. Muska, was on his way to help his fellow volunteer firefighters, but wasn't there yet when the plant exploded.

WSJ: About 14 Killed in West, Texas, Fertilizer Plant Blast

Closer to home, volunteer scuba enthusiasts in Sydney work alongside university scientists to replant crayweed, a sea plant that serves as habitat for abalone, lobsters and other fish.

BBC: Underwater volunteer vacations