This formation is characteristic of the karst. A doline or sinkhole is a closed depression draining water underground in karst areas. It can be cylindrical, conical, bowl- shaped or dish-shaped. The diameter ranges from a few to many hundreds of meters. The name doline comes from dolina, the Slovenian word meaning valley. So this was originally a colloquial Slovenian word that is now used by geologists to describe a geologic feature.
Rainwater dissolves carbonic gas (CO2) and give carbonic acid. When it infiltrates into the soil, it dissolves the limestone (calcium carbonate, CaCO3), which makes a depression. Once limestone is dissolved, there remains its impurities, clay, which makes a waterproof layer. This is why the vegetation in dolines is always greener than the landscape around. Dolines are often cultivated as this is a place keeping moisture, which is a rare thing in waterless karst areas. In some occurrence, there can even form a small pond. Man can help this process and mason the bottom, making a watering place for cattle. In the French karstic Causse regions (around Roquefort, for example), these watering places are called "lavogne". Look at my Roquefort page to see a French lavogne. I have not found the Croat name but I am sure that some one will tell me !
On a very small road, not far from Hum, this deeply carved landscape reminds of Zabrisky point, near California Death Valley ! As we were in October, the yellow and the red of the autumn trees add some more color !