USA and Mexico Transboundary Photos People bathing and washing in the Arroyo San Pedro near the indigenous village of Tlalchiyahualica, Hidalgo. Cascadas de Tamasopo, San Luis Potosi. Young girls hauling drinking water from an underground cavern, Teyahuala, Hidalgo, Mexico. Endemic swordtails Xiphophorus birchmanni and malinche, and natural hybrids due to water pollution. Indigenous vendor preparing zacahuil, a giant tamale and Huastecan culinary specialty, Xilitla, San Luis Potosi. Las Pozas, created by English surrealist Edward James, San Luis Potosi.