Oceanography is the scientific study of the ocean—our planet’s largest and most mysterious environment. Since the ocean covers about 70% of Earth’s surface, understanding it is essential for learning about weather, climate, and life itself.

Scientists who study oceanography, called oceanographers, investigate many different aspects of the sea. There are four main branches:

• Physical oceanography explores waves, currents, tides, and how the ocean interacts with the atmosphere.
  
  

• Chemical oceanography studies the composition of seawater, including salts, nutrients, and how pollution or carbon dioxide affect it.
  
  

• Biological oceanography focuses on marine life, from tiny plankton to giant whales, and how they interact in ocean ecosystems.
  
  

• Geological oceanography examines the seafloor, underwater volcanoes, trenches, and how the ocean shapes Earth’s crust.

Although Earth is often called the “blue planet,” most of that blue is saltwater. About 97% of Earth’s water is found in the oceans and seas, making it too salty for people, plants, or animals to drink without expensive treatment. Only about 3% of Earth’s water is freshwater, but even that isn’t all available for human use.

Most freshwater is locked away in glaciers, ice caps, and permanent snow (about 69%). Another large portion is stored underground in aquifers (about 30%), some of which is hard to reach. That leaves about 1% of all Earth’s freshwater easily available in rivers, lakes, and swamps—the sources humans rely on most for drinking water, farming, and industry. 

The largest and deepest ocean, stretching between Asia and Australia on one side and the Americas on the other. It contains the Mariana Trench, the deepest point on Earth. 

The smallest and shallowest ocean, located around the North Pole. Much of it is covered in sea ice, though it is shrinking due to climate change.

Surrounding Antarctica, this ocean is icy, stormy, and plays a big role in regulating Earth’s climate by circulating cold water around the globe.

The average depth of the world’s ocean is about 12,100 feet (~2.3 miles) deep. The Challenger Deep in the Mariana Trench is the deepest known point on Earth, nearly 7 miles (36,000 feet) below the ocean surface. It was created by the subduction of the Pacific Plate beneath the Philippine Plate. Because of its extreme depth, crushing pressure, and total darkness, it remained a mystery for a long time. Over the years, scientists and explorers have made a few daring expeditions to explore it.

The first successful descent was in 1960, when the submarine Trieste carried Jacques Piccard and U.S. Navy Lieutenant Don Walsh to the bottom. The journey took about 5 hours down and 3 hours back up, and they spent roughly 20 minutes exploring the trench. The descent was extremely risky: the pressure at that depth is over 1,000 times the pressure at sea level, enough to crush most submarines. During the dive, the viewing window of the Trieste developed a small crack, a terrifying moment for the explorers. Despite the crack, the engineers had designed the vessel with thick acrylic and steel, and it held under the immense pressure, allowing Piccard and Walsh to safely complete the dive. They became the first humans to reach the bottom of the ocean, proving that such extreme exploration was possible.

After decades without manned missions, the next notable dives happened in 2012, when filmmaker James Cameron piloted the Deepsea Challenger solo to the bottom. He collected samples and took high-resolution video, giving scientists an unprecedented look at life and geology in the deep ocean.

Since then, a few other manned and unmanned expeditions have explored the Challenger Deep, using robotic submarines to map the seafloor, study strange deep-sea creatures, and collect rock and water samples. These expeditions help scientists understand extreme environments, how life can survive under immense pressure, and the geology of Earth’s deepest ocean trenches.