Nearctic Limestone Caves and Subterranean Rivers - Code: Ne11H

While most of the habitats used by wildlife occur on the earth’s surface, a small number of specialized organisms spend their entire lives underground in caves. A cave or cavern in the most basic sense is simply a natural void in the ground. They tend to maintain very stable temperature and humidity, with large caves staying at a nearly constant temperature throughout the year. In temperate zones, caves are cooler than the surface in the summer and warmer than the surface in the winter. The climatic stability of caves increases as you move in from the entrance. 
Caves are typically divided into three zones – the entrance zone, the twilight zone and the dark zone. The entrance zone occurs immediately in the vicinity of where a cave opens up to the surface. The entrance zone receives direct sunlight, experiences a highly variable climate and often has photosynthetic plant growth. Many organisms not well adapted for life in caves will use this zone. The twilight zone lies beyond the entrance zone and receives only indirect sunlight. The twilight zone usually has significant airflow and experiences large fluctuations in humidity. The twilight zone is utilized by both cave adapted organisms as well as some surface dwelling animals. The dark zone receives no light and is the most climatically stable of the cave zones. This is the zone used by most true cave dwellers.
While there are numerous ways that caves form, most caves important to wildlife are solutional caves. Solutional caves are not only the most common type of cave, they are also the largest, most complex and typically are quite wet. Solutional caves form in rocks that are soluble in weak acidic mixtures. Typically, this means limestone but solutional caves also occur in chalk, marble and dolomite. Natural carbonic acid (H2CO3) present in groundwater will eat away at soluble rocks as it flows through fissures. Over time these fissures widen to form caves and cave systems. Dissolved calcium carbonate present in the water will often precipitate over time to produce adornments called speleothems. The most familiar speleothems are stalactites and stalagmites but other structures such as columns, flowstones and soda straws may also form. Mammoth Cave in Kentucky is the world’s longest solutional cave, with over 400 miles (650 km) of surveyed passages.
Areas of the cave system that are below the water table will be flooded. These may be seasonally flooded passages or at times, subterranean rivers. Subterranean rivers may flow underground from the surface or be entirely contained within cave systems. The two largest subterranean rivers in the world, Sistema Sac Actun and Sistema Ox Bel Ha are both found on Mexico’s Yucatan peninsula. 
While weathering of limestone and similar rocks below ground produces caves and subterranean rivers, the same processes above ground produce distinctive karst landscapes. Karst landscapes often have unique habitats associated with them, including the Neotropical Karst Forests in Mexico.