The story
From cauldron + bottom, in reference to the shape and colour as seen from the perspective of someone in the coal mine. Compare Sicilian campanaru (literally “bell tower”) in sulfur mining.
Etymology from Wiktionary, CC BY-SA 4.0.
cauldron bottom
Today it means: A large, round mass in a coal seam that forms part of the roof of a coal mine passage (usually being a fossilized root or stump); considered a hazard due to being prone to falling without warning. Full definition →