The park has two underground rivers called the Chontalcoatlán, which is eight km long, and the San Jeronimo, which is 12 km long. The tunnels that have been formed by these rivers are still completely active, as the water continues to cut away little by little at the rock. Both have areas with rocks and sandy beaches on each side and the darkness is complete in much of the tunnels' lengths. The San Jeronimo River has its origins in natural springs that lie in San Pedro Zictepec, Mexico State. The walls of the cavity in which it flows are up to eighty meters high. It is possible for visitors to explore this river, which requires about seven hours to do so; however, it is the easier of the two to explore as it is more level. The waters of the Chontalcoatlán originate from the Nevado de Toluca. Its descent underground has carved out an entrance fifty meters high. It is also possible to visit this natural tunnel although it is necessary to get to the entrance by vehicle. It is relatively easy to travel to a point called La Caraboya or La Ventana. However, after this point, the river descends steeply and those not in good physical condition are not advised to continue. Just after the two rivers reemerge above ground, they join to form the Amacuazac River, which is a tributary of the Balsas River.