accumulated accumulations aged calcium-carbonate candle-shapes caves ceilings coloring colors colouring colours columns crystallisation crystallise crystallization crystallize deposits distinctive edge edges floors formation forms icicle-like minerals old Particles pillars pointed shape shaped shapes Singapore slow slow-growth slowly smooth Stalactite stalactites stalagmites subterranean surface surfaces texture textures unusual white cave cavern caving compounds geological geology limestone mineral-deposits speleology speleothems spelunker spelunkering Underground