Expanding Mexico City's Sewage System

As a public health major, this report about sewage systems in Mexico City, the world's second largest city, caught my eye. After listening to it, I could not believe that in a city with more than 20 million people, there are floods of sewage so high that sometimes cars cannot drive. I find it difficult to believe that there are still open sewage canals in such a booming city and that they haven't found a solution to this problem earlier.