 More than of gasoline seeped from a corroded Mexican Petroleum (Petróleos Mexicanos—Pemex) pipeline into the municipal sewer system, where it combined with gases and industrial residuals to produce a massive explosion that killed 190 persons and injured nearly 1,500 others.Mexico City confronts authorities with perhaps their most daunting environmental challenge.Geography and extreme population levels have combined to produce one of the world's most polluted urban areas.