cave caves caving crystal crystals rock rocks Stalactite stalactites cavescape geology china Fengdu karst stalagmite stalagmites stone cavern deposit drip formation Mineral water accumulated accumulations aged calcium-carbonate candle-shapes ceilings coloring colors colouring colours columns crystallisation crystallise crystallization crystallize deposits distinctive edge edges floors forms icicle-like minerals old Particles pillars pointed shape shaped shapes Singapore slow slow-growth slowly smooth subterranean surface surfaces texture textures unusual white