C．you are interested in the magical world D．you enjoy reading the eight-book series

　　C

　　A blue hole is a flooded sea cave with a hole that opens up at the land's surface. These cave systems form in carbonate rock, often on islands. Some blue holes have very special rock formations and water chemistry. Far below sea level, they contain some of the harshest environments on Earth, with no oxygen and no light. Yet these areas are filled with life forms that have adapted to the extreme conditions.

　　Information gathered from these blue holes is helping scientists to increase their understanding of biology, archaeology and geology. But exploring these blue hole environments brings danger as well as discovery.

　　Blue holes get their name from the color some have when seen from the air. The color is usually a reflection of the sky on the water. But not all of these cave systems have blue surfaces. Some contain dark or muddy water.

　　Blue holes are the result of erosion in which water breaks down rock. Rain falling thousands of years ago contained chemicals which slowly wore away at the limestone landmass. These holes were later filled with sea water as the sea level changed. The rising and falling of sea levels and the mixture of salt and fresh water further wore away at these cave formations. Blue holes are vertical caves. But they can also have horizontal cave formations that may be hundreds of meters long.

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