Cave A natural open space within a rock formation large enough to accommodate a human. A cave may have an opening to the outside, is always underground, and sometimes submerged. Caves commonly occur by the dissolution of soluble rocks, generally limestone, but may also be created within the voids of large-rock aggregations, in openings along seismic faults, and in lava formations. Groundwater Sites Adapted from USGS Site Types. http://wdr.water.usgs.gov/nwisgmap/help/sitetype.html ODM2 SiteType Controlled Vocabulary ODM2 Working Group A vocabulary for describing the type of a data collection Site. To some extent, these types represent the ultimate feature of interest that the site was established to measure. For example, a Stream Site was established to measure properties of a Stream.