One of the ironies of human civilization is that we tend to destroy the things that we most depend on and cherish. The ozone layer, rainforests, rivers, and lakes--you name it, we've probably done some damage. So it's really no surprise to learn that the planet's coral reefs are in danger.
Reefs can be massive, stretching for miles in warm, tropical waters near the equator. They are created by colonies of tiny creatures called polyps. When polyps die they leave behind their dwellings: the hard, branching limestone structures we know as coral.
There are three basic kinds of coral reefs. Fringing reefs grow on continental shelves right along the coastline--for example along the Western coast of Africa. Barrier reefs such as Australia's Great Barrier Reef exist farther out from the coast. Since they parallel the coastline, such reefs form barriers for ships heading for deeper waters. Finally, coral atolls first form around volcanic islands. When the volcano eventually sinks into the ocean, only the reef remains as a ring in the sea.
Natural events such as hurricanes can damage coral reefs, but humans pose the biggest threat. From sewage runoff to shipping and over fishing, human expansion and activity can be lethal. For example, fuel and oil leaked from ships pollute the water, which in turn pollute reefs and hamper coral reproduction.
Although some reefs sustain a healthy tourist trade, it can be a double edged sword. When enough tourists pocket a piece of reef as a souvenir, the damage can be extensive. Coral reefs are truly wondrous to behold, but if we're not careful they won't be around much longer for us to enjoy.