Ключевые слова: adventure, cave, cavern, dark, depth, digging, earth, earth sciences, engineering, excavation, exploration, geologic, geology, historic, industrial, industry, infrastructure, mine, mine shaft, minecart, mineral exploration, mineral resources, mineralogy, minerals extraction, mining, mining equipment, mining industry, natural resources, ore, ore extraction, rail line, rail transport, railway, rocky, rugged, spelunking, subterranean, track, transportation, tunnel, tunneling, underground, underground passage,