<p>This Special Issue is devoted to the study of caves and other subterranean sites with 25 or more specialized species (stygobionts and troglobionts) in a geological and hydrogeological context. These investigations including those in the previous Special Issue deal with 26 of the 30 known sites.&nbsp;These hotspots are widely distributed in temperate and subtropical zones but are few in the tropics. A concentration of hotspots can be found in southern Europe. Contrastingly no known hotspots have been located in Africa.</p>