From the World Heritage inscription:
Sian Ka’an is situated on the eastern side of the Yucatán Peninsula in the state of Quintana Roo. Where possible, boundaries were defined to coincide with natural features: the site is bounded by the Caribbean Sea and the barrier reef to a depth of 50 m in the east; by the junction between the marshes and semi-evergreen forests in the south-east; and by the junction of Chetumal and Espiritu Santo bays catchment basin in the south. The northern and north-eastern boundaries are defined by the limits of farming cooperatives. The northern sites can be reached by a dirt track from Tulum, whereas Punta Pajaros is only accessible by boat or aircraft.
Sian Ka’an lies on a partially emerged coastal limestone plain which forms part of the extensive barrier reef system along the eastern coast of Central America. Much of the reserve lies in a zone of recent Pleistocene origin which still appears to be in a transitional stage. A large series of sinkholes (cenotes) exist in the area and are characteristic features of the Yucatán and Florida peninsulas. The hydrological cycle is complex and the water table is permanently close to the surface. There is little surface running water within the reserve as it usually filters fairly rapidly through the shallow rendzina and saskab (granular whitish and brittle limestone) soils, and the limestone rock to subterranean channels. Owing to their hardness the waters in the reserve are generally very clear. A geological fault crosses the reserve from south-west to north-east, influencing its topography and hydrology. In general, soils are unsuitable for agriculture.
Most people who travel to the Riviera Maya region of Mexico probably go to visit is famous beaches or Mayan ruins. They are probably unaware of the ecological treasure located nearby in Sian Ka’an Biosphere Reserve.
My day in Sian Ka’an was spent cruising through mangrove swamps in an outboard boat, photographing birds and exploring old Mayan structures.
If you should visit Tulum, Playa del Carmen or even Cancun I’d strongly suggest taking a trip away from the tourist hotspots for half a day and visit Sian Ka’an. Its a unique place to explore a mangrove ecosystem and it will give you a chance to see what the region looked like before the arrival of Europeans.
View my complete list of UNESCO World Heritage Sites.