El Cielo stretches between tow spurs of the Sierra Madre Oriental in south-western Tamaulipas. A dramatic range of elevation and a favourable location near the Tropic of Cancer produce vastly distinct ecosystems, including tropical, cloud, pine-oak and evergreen forests. Designated a biosphere reserve in 1986, the park is a haven for wildlife, while its cloud forest provides a rainwater catchment for the tropical forest and commercial agriculture to the east and southeast.

The reserve has a high level of biodiversity. There are 95 species of mammal, including endangered wild cats, white-tailed deer, and black bear. Additionally, there are about 60 species of reptiles and 430 of birds.