During my travels, I've visited two major caves: Mammoth Cave in Kentucky and Carlsbad Cavern in New Mexico.
Mammoth Cave is the longest known cave in the world - over 390 miles have been explored. Carlsbad Cavern is smaller but contains a staggering array of speleothems, the beautiful formations that appear from steadily dripping water. (The most commonly known speleothems are stalagmites and stalactites, although there are many other forms.)
I don't much like being underground and watched my mind make up stories about what would happen if one of the cave ceilings were to collapse. These fears were real, but not true - not in alignment with the reality that these cave ceilings haven't moved for thousands of years.
So these journeys under the earth made for good practice.
I also noted, especially in Carlsbad Cavern, with its extraordinary range of speleothem formation, that the earth is actually quite fluid - despite the apparent solidity we experience on the surface.
Under ground, the inherent plasticity of our planet becomes much more obvious, with water dripping everywhere, stalagmites continuing to reach toward the cave ceilings, and curtains of calcium rippling in stone.
It was a good reminder of the limits of experience. And an even better reminder that everything always changes.
Photos taken in Carlsbad Cavern with an iPhone 4S using available light. Click on images for a better view.