Foiba

A foiba (plural: foibas or foibe) — jama in South Slavic languages scientific and colloquial vocabulary (borrowed since early research in the Western Balkan Dinaric Alpine karst) — is a type of deep natural sinkhole, doline, or 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 hectares. They are common in the Karst (Carso) region shared by Italy and Slovenia, as well as in a karst of Dinaric Alps in Bosnia and Herzegovina, Montenegro and Croatia.

Grotta Plutone is a foiba close to Basovizza, Trieste (Italy). It's a deep natural sinkhole with a overhanging entrance typical of the Karst Region. This image is part of the photographic project "Foibe", 2016 made by Sharon Ritossa
Simple scheme of a foiba