This thermal feature did not exist before the winter of 1947-1948, and it was first discovered on June 10,1948. Thermal activity may have shifted from Sour Lake to form Black Dragon's Caldron. When it was discovered, a crater about 43x68 feet existed at the northern end of the present pool. Since then, eruptive activity has slowly moved south 200 feet along a natural crack. Hydrogen sulfide rising within the spring has combined with oxygen to form sulfuric acid, creating a hostile environment for plant life. Iron sulfides are responsible for the black color of the caldron.