Drip…. Drip…. Drip….
Two things happen with each drip: (1) the water falling from the roof of the limestone cavern, from the carbuncle-sized protrusion pointing towards the floor, leaves some of its soluble calcium carbonate behind; (2) the water hitting the tiny finger of wet stone on the floor of the cavern gets a brand new coating of fresh calcium carbonate added to all the previous layers that have landed there, a micron-thin slice of water with a few accreting granules of calcium carbonate each drop.
Drip…. Drip…. Drip….
Another drop, then another. Over the course of the year, perhaps 0.05 millimeters (mm) will be added to the ceiling pimple of wet stone, and another 0.05 mm to the floor finger. Or maybe there is a good bit of flowing water in this cave and the ceiling and floor protrusions will grow up to 3 mm a year. The average growth rate is estimated at about 0.13 mm/year. At that rate, the tiny finger will push skyward (although the only sky it may ever know is its mirror image growing down from above) slowly, ever so incrementally, until in in 164,123 years it has grown to the size of the tallest stalagmite currently known in the world, a stalagmite 70 feet in height located in Vietnam.
Cut a stalactite or stalagmite open and you see the additive process displayed – a portrait of how it came to be, much like the rings of a tree show its age, although in the case of these cavern formations, it shows patterns of fast and slow growth, changes in mineralization of the water, droughts and floods, a time-lapse photograph of what used to be and what is. In this phenomenal cross-section of a stalactite, the water was imbued with manganese carbonate (rhodochrosite), a mineral that often has a pink or red color. At some point in the distant past, this was a cluster of tiny white pimples pointing towards the cavern floor when, for some reason, the mixture of carbonates changed to favor manganese, the water continued to drop ever so slowly, and this family of stalactites was frozen in time.
Of course, these are simplifications; stalactites and stalagmites sometimes grow briefly, then the water course diverts and they remain short for the rest of time, or they grow in sheets and curtains if the water spills like a slow waterfall seeping through the earth above. These fascinating growths take their time, though, growing drop-by-drop, fractional bit-by-bit, until they are magnificent in their dark realms, secret until found by lanterns and humans poking through a slot in a wall behind a curtain of shrubs.