The Challenger expedition of 1872 to 1876 is where modern oceanography began. It was the first voyage to explore the deep sea successfully on a global scale. It observed currents, water temperatures, life and the seafloor.

On 23 January 1960 Don Walsh and Jacques Piccard rode the bathyscaphe Trieste down to the Challenger Deep, the first people to reach the deepest known point of the ocean. As of November 2026, the Challenger Deep is about 11 kilometers (almost 7 miles) down.

The longest mountain range on Earth is under the sea. It is called the mid-ocean ridge. It is a chain of undersea mountains about 40,000 miles (65,000 kilometers) long that winds around the whole Earth. Mid-ocean ridges form where tectonic plates move apart. Hot rock rises from the mantle, melts, and cools into new seafloor.

Hydrothermal vents were found in 1977 near the Galápagos Islands. They are openings on the seafloor where water heated by magma flows out. Life there runs on chemical energy instead of sunlight.

Sunlight, temperature and pressure all change with depth. Light fades out below about 1,000 meters, temperatures fall toward 4 degrees Celsius, and pressure rises all the way to the bottom.

Zooplankton rise to the surface at night to feed on phytoplankton and sink to deeper water by day. This daily trip is the largest migration on Earth, and it can be seen from space.

Upwelling brings cold, nutrient-rich water up from the deep. These nutrients help plankton grow, so good fishing grounds are often found where upwelling is common.

A tsunami is a set of giant waves, most often made by an earthquake under the sea. A tsunami in deep water can move up to 800 kilometers (500 miles) per hour. In shallow water it slows down and can rise to 20 to 30 meters (65 to 100 feet) before it hits land.

Hurricanes get their energy from warm sea water. Heat and moisture from the warm ocean surface rise into the storm. It keeps strengthening as long as it stays over warm water. It must also not be torn apart by high-altitude winds.

Sea level is rising mainly because warm ocean water expands and because ice on land melts and runs into the sea.

As of November 2026, most of the deep ocean has never been seen by people.

Words to know

• oceanography: ocean science
• bathyscaphe: deep diving craft
• hydrothermal: hot water
• upwelling: water rising
• zooplankton: drifting animals