Adamite arsenate calcite crystal Durango limonite Mapimi Mexico Mina Ojuela Mineral Zinc bottom pebble river rock rocks sand spring stone stones Underground water wet abyss acid acidic Campi Flegrei chemical chemistry condensation crater crest crevice cristals damp dangerous devil flowers fracture fumarole fumes geologic geology heat hell hot hydrogen landscape minerals nature polution Pozzuoli smell smelly solfatara steam sulfur sulfuric sulphur sulphuric unpleasant vent volcanic volcano vulcano yellow Dirt earth environment igneous metamorphic object science Sed Soil