An underground tank releases vapors depending upon air pressure. As pressure increases, vapors decline. When pressure decreases, vapors increase and escape the tank or spill to pollute the atmosphere. A container for trapping vapors has a housing upon a base. Within the housing one or more bags hang upon stems connected to piping at fittings. The stems have perforations to admit vapors into the bags. The stems are connected serially to the piping upon the base. As vapors enter the piping, the bags inflate within the housing. When tank or spill pressure declines, the vapors exit the bags and return to the tank or spill via the piping.