Ajusco is the southern part of Mexico City, beyond its light rail. It's home to 2 significant mountains. Ajusco is the highest point of Mexico City at almost 4000 metres in height. Xitle, on the other hand, is an extinct volcano that had only ever erupted once, and destroyed the city-state Cuicuilco, today the oldest surviving structure in North America.
To get to Santo Tomas Ajusco, you get get to Aztec Stadium and take a bus (MXN$7) for around an hour and a half.