A cave is any kind of natural, hollow, underground passage or enclosure with an opening to the surface. Caves are most commonly found in what's called a karst landscape, characterized by sinkholes, substantial underground aquifers and active subterranean drainage. Around 20 percent of the United States qualifies as karst [source: National Caves Association].
Caves are formed by four basic processes:
- The majority of caves are limestone caves. These form when rainwater seeps down through the soil, picking up extra carbon dioxide. The result is a weak acid called carbonic acid. The water collects in underground aquifers where it slowly eats away the limestone. Some limestone caves are also formed by large amounts of running rainwater which carves away at the rock in a process called corrasion, or erosion by abrasion [source: Nova].
- Only very recently have researchers discovered that microscopic bacteria have helped form some of the most impressive caves in the world. Bacteria called extremeophiles (creatures that thrive in "extreme" or highly toxic conditions) feed off of deep underground oil deposits. After eating a big meal, they expel hydrogen sulfide gas that bubbles up through the groundwater, picking up extra oxygen to become sulfuric acid, a powerful corrosive agent. Other bacteria live in the caves themselves, feeding off hydrogen sulfide to make even more sulfuric acid.
- Pounding waves eat away at weak points in seaside cliffs forming caves with large overhanging ceilings. Most sea caves are carved from sandstone or limestone.
- Lava tubes are created when the sides of an active lava flow cool first, forming a crust that slowly merges to cover up the still-liquid center. When all the liquid lava flows out, only the hardened tube is left. If part of the tube ceiling collapses, you have an opening to a lava cave. [source: Nova]
Stalactites (the ones that hang down) and stalagmites (the ones that point up) are the best-known examples of speleotherms, also called cave formations. There's an astounding variety of speleotherms, some with oddly descriptive names like fried eggs and bacon. When carbonic acid eats away at limestone, it creates the mineral calcite, which is then carried by rainwater into the cave. When this rainwater drips onto the floor of a cave, it very slowly deposits bits of calcite until a stalagmite is formed. Other speleotherms are formed by calcite being precipitated in brilliant patterns and crystalline formations on cave walls, ceilings and floors.
There are three different classifications of animals that live in caves:
- Trogloxenes are temporary visitors that live in and around the entrances to caves. Bats are the most famous trogloxenes, but the list also includes bears, bobcats, raccoons, cave swallows and pack rats.
- Troglophiles spend most of their lives in caves, but occasionally wander out for food. Examples would be salamanders, crickets, flatworms and different species of spiders and daddy longlegs.
- Troglobites live their entire lives in the pitch-black depths of dry and sea caves. All are blind and some don't even have eyes. Examples are the Ozark blind salamander, the Tooth Cave spider and Tooth Cave beetle, cave fish and blind shrimp [source: U.S. Geological Service]
Cave conditions vary dramatically depending on how the cave was formed and where it's located. Some caves are wet and muddy; others are dry and dusty. The temperature of a cave is very stable. It's always the average annual temperature of the surface above the cave [source: National Speleological Society].
But why would anyone want to crawl around in a big hole in the ground? Read on to find out.