Caves
A cave or cavern is a natural underground space large enough for a human to enter. The term applies to natural cavities some part of which is in total darkness. The word cave also refers to smaller spaces like rock shelters, sea caves, and grottos.
Speleology is the science of exploration and study of all aspects of caves and the environment which surrounds the caves.
![Lechuguilla_Cave_Pearlsian_Gulf []](http://www.travelet.com/wp-content/uploads/2010/02/Lechuguilla_Cave_Pearlsian_Gulf-.jpg "Lechuguilla_Cave_Pearlsian_Gulf []")