On October 23, 2015, Southern California Gas Company workers discovered a leak in one of the over 110 wells at the Aliso Canyon natural gas storage facility, about one mile north of homes in Porter Ranch. The gas blowout began spewing 110,000 pounds of methane per hour. The blowout involved gas stored under pressure in an underground reservoir; the stored gas included mercapatan (tert-Butylthiol), an odorant added to the odorless natural gas to produce a "rotten egg" smell for safety. The California Air Resources Board estimated that the leak increased California's methane-gas emissions by 25%.