If you've ever seen a movie or television show that takes place in a tropical paradise, you may have seen people swimming in a shallow pool of water that's separated from the larger ocean by coral reefs or small barrier islands. That's a lagoon!
Of course, lagoons aren't limited to just tropical areas. They're common features along coastlines all over the world. Anywhere there's a shallow body of water separated from a larger body of water by a reef or an island, you've got yourself a lagoon. In fact, the word “lagoon" comes from the Italian laguna, which describes the waters around Venice.
Although some people include bodies of fresh water in the definition of lagoon, others only apply the term “lagoon" to bodies of salt water. If a body of water that might otherwise be a lagoon receives a regular inflow of fresh water, it most likely will be called an estuary.
Just because a body of water fits the definition of lagoon doesn't mean it'll have “lagoon" in its name. Pamlico Sound in North Carolina, Great South Bay in New York, Banana River in Florida and Lake Illawarra in New South Wales are all technically lagoons despite their names.
Lagoons tend to form along coastlines with a gentle slope. This makes most lagoons shallow and sensitive to changes in sea level. If the sea level drops, the lagoon may dry up. If the sea level rises, the protective reefs or islands may end up underwater.
Some of the most beautiful lagoons occur in tropical areas near coral reefs. When these lagoons happen to be relatively deep and are surrounded by ring-shaped coral reefs, scientists call them atolls.