My history sleuthing: https://davehuer.com/cartoproblematica/
Subject: Karst, caves, and associated historical anomalies
What is Karst ?
Images: sourced from cancaver.caKarst, a Complex Landscape Sculpted by Water
“Karst is a distinctive topography in which the landscape is largely shaped by the dissolving action of water on carbonate bedrock (usually limestone, dolomite, or marble). This geological process, occurring over many thousands of years, results in unusual surface and subsurface features ranging from sinkholes, vertical shafts, disappearing streams, and springs, to complex underground drainage systems and caves.”
Karst diagram: cancaver
Photos: #1: cancaver; #2-6 D.Huer & caver friends