PLANET EARTH Caves
As soon as its nest is removed a bird will immediately build another. So, as long as this valuable harvest is properly controlled, the colonies will continue to flourish. These Bornean caves are among the biggest in the world and they're still getting bigger as each year rainwater eats away a little more limestone. But water in caves doesn't only erode. It also builds.
This water is loaded with dissolved limestone and when it meets the air in the cave some of that is deposited as a mineral - calcite（方解石）. As it builds up so the calcite forms decorations that hang from the ceiling - stalactites（钟乳石）. Each drop leaves behind only a miniscule amount of calcite, but over time the process can produce some spectacular results. If the water seeps through the ceiling quickly, then the calcite is deposited on the floor of the cave and that creates stalagmites.
Variations in water flow and the air currents produce an infinite variety of forms, but all are created by the same process - the slow deposition of dissolved limestone. And when stalactite meets stalagmite a column is born. Structures like these in North America's Carlsbad Cavern（卡尔斯巴德洞穴） can take many thousands of years to develop. But sometimes the formations in a cave stop growing altogether.
