THE THIRD LARGEST of the world’s oceans, the Indian Ocean covers 28 million sq miles (73 million sq km) and contains some 5,000 islands, many of them surrounded by coral reefs. This ocean is unique because, unlike the Atlantic and Pacific, it has no outlet to the north. It contains both the saltiest sea (the Red Sea), and the warmest sea (the Persian Gulf) on Earth. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by the Southern Ocean or Antarctica, depending on the definition in use. Along its core, the Indian Ocean has some large marginal or regional seas such as the Arabian Sea, the Laccadive Sea, the Somali Sea, Bay of Bengal, and the Andaman Sea. The Indian Ocean is at risk from pollution, especially from oil tankers leaving the Persian Gulf. Monsoon rains and tropical storms can bring disastrous flooding to its northern coasts.
It is a vital trading hub, connecting the Middle East to Southeast and East Asia, as well as Europe and the Americas. … At the heart of the geopolitical struggle in the Indian Ocean is the ability to sustain a military presence near the key choke points connecting its trade routes. It is the only ocean with an asymmetric and, in the north, semiannually reversing surface circulation. It has no separate source of bottom water (i.e., the Indian Ocean’s bottom water originates outside its boundaries) and has two sources of highly saline water (the Persian Gulf and the Red Sea).
Water temperatures of the Indian Ocean range between 66 and 82 degrees Fahrenheit (19 to 30 Celsius) on the ocean’s upper layer. Since it does not connect to the Arctic Ocean, the coldest ocean on Earth, the Indian Ocean stays pretty warm all year round.
The waters of the Pacific Ocean comprise the world’s largest heat reservoir, by far, and it is the warmest ocean, overall, of the world’s five oceans. Of the five ocean basins, the Atlantic Ocean is the saltiest. On average, there is a distinct decrease of salinity near the equator and at both poles, although for different reasons.

The islands of the Indian Ocean include coral atolls, like the Maldives and Seychelles, that attract thousands of tourists every year. Although this brings money to the islands, it also threatens to damage the environment. Gradual erosion of the coral reefs also leaves the islands exposed to ocean tides and flooding. This Maldive island has a barrier to protect it from sea damage.

Coral is formed in warm waters by tiny creatures known as polyps. These marine creatures build limestone skeletons around themselves. Over many thousands of years, these skeletons gradually grow up toward the surface of the ocean to form a coral island. An atoll, shown right, is a form of circular coral reef that grows around an underwater volcano. As the volcano sinks, the coral forms an atoll. The water in the center is called a lagoon.

More than 200 large ships a day sail around the Cape of Good Hope at the southern tip of Africa as they enter or leave the Indian Ocean. Many are vast tankers laden with oil from the Persian Gulf. Smaller ships are able to pass through the Suez Canal.
On the other side of the Indian Ocean, ships pass through the Strait of Malacca carrying cargo to ports in eastern Asia.

Oceans are salty because minerals dissolved from rocks by rivers are washed into them. Around the shores of the Indian Ocean, people extract the salt by channeling water into shallow pans. The Sun’s heat evaporates the water, leaving salt behind, as shown here in Mauritius.

Madagascar lies off the east coast of Africa and, because of its isolation, is home to many unique plants and animals. Most people in Madagascar scratch out a living by farming, clearing a new patch of land each year to plant their crops. One of the island’s main crops is vanilla, grown for use as flavoring in food and drinks. Women traditionally have elaborate hairstyles that indicate the village they are from.