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In the state of Morelos (where Cuernavaca is located), there is a little paradisiac oasis, it's a fresh water spring -well, more than fresh, sometimes gelid, as the water comes from the Popocatepetl ice melting-.
It's not far from Mexico city and you can do a day trip to get away from the pollution and traffic.
There are many areas where you can enjoy the lazy river created by the spring. This is a nice place to do open water practices for scuba diving
There is a rustic hotel and camping area. They also rent the scuba equipment for certified divers.