active danger deposits geothermal geothermally hot Mineral natural new zealand rock Rotorua sulfur sulphur toxic vapor vapour accumulated accumulations aged calcium-carbonate cave cavern caving ceilings coloring colouring columns compounds crystallisation crystallization crystallize distinctive edges formation forms geological geology icicle-like limestone mineral-deposits minerals old Particles pointed shape shaped shapes slow slow-growth slowly smooth speleology speleothems spelunker spelunkering Stalactite stalactites subterranean surface surfaces texture textures Underground unusual deposit Geological phenomenon outcrop yellow