A company called Envirotech collected them for ten years and stored them in part of the Sonoran Desert National Monument. The idea was to recycle them into fuel. Unfortunately for the company, the state of Arizona decided to shut them down for multiple fire code violations. These include obstructed fire roads and an on-site fire-protection water system that didn’t meet minimum requirements. Unfortunately for everyone else, if ignited, the piles could “burn unhindered for over ten years” according to the Arizona Attorney General’s Office.
All told, about three billion used tires are stockpiled across the United States. All the more reason to hope that a microorganism eventually emerges that can metabolize vulcanized rubber.