1 . A blue hole is a special kind of underwater cave found inland or in the sea, which forms when the earth above a cave falls in and water fills the space.

An inland blue hole’s water is very still and has different layers. A layer of fresh rainwater floats on top of salt water; the fresh water keeps oxygen from the atmosphere from reaching the salt water; brightly colored bacteria live where the two layers meet.

Diving into blue holes is very dangerous. Near the top of the blue hole, there is a layer of toxic gas, which causes itching, headache, and—in large amounts—death. Divers must also be fast. They have to get in and out of a cave before their oxygen runs out. Additionally, divers have to follow a guideline as they swim through a blue hole because it is very dark inside. Without the guideline, they may get lost.

If blue holes are so dangerous, why do explorers and scientists risk their lives to explore them? The reason is that these underwater caves can provide valuable scientific information. They provide clues about geology, archaeology, and biology. For example, some blue hole creatures probably haven’t changed for millions of years.

The blue holes could even provide clues about astrobiology. For example, divers have found bacteria there that can live without oxygen. Astrobiologist Kevin Hand says the bacteria may be similar to forms of life that might exist on Jupiter’s fourth largest moon, Europa. “Our study of life’s extremes on Earth,” he says, can help increase “our understanding of habitable environments off Earth.”

In addition, the oxygen-free environment of the blue holes preserves bones of humans and animals that fell into the caves long ago. By studying blue holes, we can understand what life was like in prehistoric times. As cave diver Kenny Broad says, “I can think of no other environment on Earth that is so challenging to explore and gives us back so much scientifically.”