1) Karst formation 
Foiba (plural: foibas or foibe) is a type of deep natural sinkhole, doline, sink and is a collapsed portion of bedrock above a void. Sinks may be a sheer vertical opening into a cave, or a shallow depression of many acres which are common in the Kras (Carso) region, a karstic plateau region shared by Italy, Slovenia and Croatia. ==History== The te...