Not all is green in Mexico City's Aztec garden district Colorful gondolas lazily ferry tourists around the floating gardens of Mexico City's Xochimilco district, but not all is green in this idyllic vestige of the Aztec empire increasingly choked by urbanization. Today, the canals of Xochimilco are threatened as this metropolis of nine million people struggles to deal with the pressure exerted by its booming population. IMAGES AND SOUNDBITES