Natural Object Type Types of natural hydrogeological objects. english eng CodeListValue spring valid Any natural situation where groundwater flows to the surface of the earth. Thus, a spring is a site where the aquifer surface meets the ground surface. CodeListValue seep valid A moist or wet place where groundwater reaches the earth's surface from an underground aquifer. CodeListValue swallow hole valid A natural depression or hole in the Earth's surface, also known as a sink, shake hole, sinkhole, swallow hole, swallet, doline or cenote, it is mostly caused by karst processes - the chemical dissolution of carbonate rocks or suffusion processes for example in sandstone. CodeListValue fen valid Low land that is covered wholly or partly with water and that usually has peaty alkaline soil and characteristic flora (as of sedges and reeds). CodeListValue not specified valid Unspecified places where groundwater meets the surface. CodeListValue other valid Other places where groundwater table meets the surface. CodeListValue