An ocean is a large body of salt water that covers more than two-thirds of Earth’s surface. It plays a critical role in providing food, oxygen and shelter for many forms of marine life, including microscopic plankton and fish, marine mammals, coral reefs, and other plants. Oceans also provide a vital source of transportation and trade, and they support a wide range of human activities, such as fishing, offshore oil drilling, and mining.
An estimated 97 percent of the world’s water is in the ocean, and it influences weather patterns, temperature, and the supply of food to all living things. Despite its enormous size and impact on every aspect of life on Earth, most of the ocean remains a mystery to scientists. Only a small fraction of the ocean has been mapped or studied, and even less of its depths have been explored by humans.
The difference between the sea and the ocean is not just a matter of size: a sea may be surrounded by land while an ocean is always surrounded by other oceans. A sea can also be a region within an ocean, such as the Channel Sea, North Sea or the Caribbean Sea.
The oceans are constantly changing, forming and dissolving, but some of their characteristics have remained the same over millions of years. For example, ocean water is salty, which is due to the fact that a portion of rock on the surface of the Earth is constantly thrust into the sea by the movement of tectonic plates. This process, known as “seafloor spreading,” creates vast underwater mountain ranges called mid-ocean ridges.
Oceans absorb carbon dioxide from the air, which makes them more acidic. This phenomenon, known as ocean acidification, erodes the shells of marine animals and changes the composition of the seafloor from white calcium carbonate to red-brown mud. Ocean organisms have evolved over time to live in a certain level of acidity, but the rapid rise of carbon dioxide is creating an unnatural ocean pH that cannot be buffered in the same way it has been in the past.
One of the biggest challenges facing oceanographers today is how to explore the depths of the ocean, which are currently beyond our reach. Scientists like National Geographic Explorer Marcello Calisti are developing innovative ways to do just that, such as by mimicking the way octopuses move through the water. Other technologies used to explore the ocean include remotely operated vehicles (ROVs) and satellite technology. Eventually, robots will be able to go far deeper into the ocean and provide us with a more complete picture of its extraordinary biodiversity. They will also help us understand the changing conditions of our own planet’s largest and deepest body of water.