Sink holes is a depression or hole in the surface of the earth. i.e. The surface can sink into a hole and this can happen suddenly.
sinkholes are common where the rock below the land surface can naturally be dissolved by circulating ground water. As the rock dissolves, spaces develop underground. These sinkholes are dramatic because the surface land stays intact until there is no support below which causes the land to sink. A sudden collapse of the land surface occurs.
A variant of this is the blue hole. This is a sink hole in water or sea. There are also called vertical caves. The dissolved oxygen levels in such holes are very low, hence doesnt support any life, however some forms of bacteria can survive.
Blue holes can be very spectacular if they are found in areas having coral reefs. Such blue holes can become tourist destinations. There are many different Blue Holes located around the world Belize, Bahamas, Red Sea etc.--
