what are blue holes? 

INTRoduction    

     A blue hole is a marine sink hole that was formed by weathering and erosion of limestone during the ice age.

     In the ice age the sea level was 330-390 feet lower than it is now. When the water level rose because of melting ice, the holes filled up with water. This created the blue holes that we have today.

Famous Blue Holes

     Many examples can be found around the world , for example, the Blue Hole of Dahab in the Red Sea, the Great Blue Hole of Belize and The Dragon Hole in the South China Sea. 

     The Great Blue Hole is the only blue hole that can be seen from space. 

     These are popular places for experienced scuba divers to visit, but they can be very dangerous. The Blue Hole of Dahab is a very, very dangerous blue hole to dive in. 130 to 200 divers lost their lives while diving there in the last 10 years.

the layers

     Blue holes consist of four layers that do not mix. These are:

1. The freshwater layer is made of  rainwater and sits on top of the denser salt water.

2. The ancient salt water layer.

3. The layer of toxic hydrogen sulfide made by bacteria.

4. The layer of anoxic salt water.

Conclusion

    I think blue holes are cool. They are a unique part of our ocean ecosystem. I think that the different layers are very interesting, especially the fact that they don't mix together.