These flooded cave systems can be found in the ocean, or they can be found inland. Ocean caves are affected by tides, so they always have water movement. But blue holes on land are very still. They have several layers of water, chemicals, and bacteria. The top layer of fresh water comes from rainfall. This layer acts like a cap on top of the layered mixture, and keeps out oxygen from the atmosphere. The fresh water floats on a denser layer of saltwater. Underneath this is a layer of poisonous hydrogen sulfide, produced by bacteria living in the water. Underneath this layer is anoxic seawater -- water that does not contain any oxygen.
Underwater caves are probably one of the least explored ecosystems on the planet. One reason blue holes have not been fully explored is that they can be extremely dangerous. There are many safety rules that divers must follow to help ensure their survival.
First, divers must have training and experience to swim in these caves. Those who explore a cave for the first time must establish a thin rope called a guideline. This line helps them to safely enter and bring more than one set of breathing equipment in case one device fails. Meanwhile, they must pay careful attention to their air supply.
The rule is to use a third of their air to enter the cave, a third to exit, and a third for emergencies.
For most explorers, though, the possibility of discovery in these cave environments makes the experience worth the risk. You can jump into what looks like an insignificant little hole in the ground, and come out with information that's of value to many different disciplines, Scientists are interested in these caves because oxygen-free conditions there are similar to those on Earth long ago, before oxygen existed on our planet., as well as similar to space.
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