Complex sentence: 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.
Split into simpler sentences: Sinks may be a sheer vertical opening into a cave, or a shallow depression of many hectares. They are common in the Kras (Carso) region, a karstic plateau region shared by Italy, Slovenia, and Croatia.