I recommend that you look for images related to Agricola's sixteenth-century treatise on mining. His book is the standard source to understand late-medieval mining in Europe, and the approach he outlined dominated underground mining until "modern" engineering techniques became the standard in the latter half of the nineteenth century. The older approach used a great deal of following mineralization/deposits, which caused excavations to meander. Whenever possible, supports relied on natural rock and were supplemented with timbers only when absolutely necessary. The older approach, as adopted by the Spanish and Mexican miners, resulted in a technique called El systema del rato, which "modern American engineers reduced - with ridicule - to "rathole mining." But it was an efficient, pragmatic approach, even if the net result were excavations that seemed at times to be "sprawling natural cave formations" - as you indicate.