The Pacific Ocean holds more than half of the free water on Earth and has the oldest ocean rocks, about 200 million years old. It is called the Ring of Fire for the earthquakes and volcanoes near its edges.

Ocean trenches form at subduction zones, where an older, denser tectonic plate is pushed under a lighter one and bends the seafloor into a steep, V-shaped depression. The Challenger expedition sounded the Mariana Trench, which gave the Challenger Deep its name.

The moon pulls the ocean into two bulges, one on the side facing the moon and one on the far side, and as Earth turns, coasts pass through the bulges, which gives high tides. The Moon is much closer to Earth than the Sun is, so it has the stronger pull on the ocean. The Sun adds a little less than half as much tide-generating force as the Moon.

Tides are very long waves. High tide is the crest of the wave reaching a coast and low tide is the trough, and the difference in height between them is the tidal range.

The Gulf Stream is a strong warm current in the Atlantic Ocean that flows north along the east coast of the United States. It moves more water than all the world's rivers combined, and it carries heat from the tropics north across the Atlantic.

Rain and rivers carry salts from rocks into the ocean. Rain slightly acidic from carbon dioxide wears away rock, and the freed ions flow to streams, rivers and the sea. Hot vents on the seafloor add some salts too, and about 3.5 percent of the weight of sea water is dissolved salt.

Dead zones are ocean areas with too little oxygen for most sea life. They can form when an algal bloom dies and the microbes that break it down use up the oxygen faster than it can be replaced.

Seagrasses are true flowering plants with roots, not seaweed. Seagrass meadows store an estimated 18 percent of ocean carbon even though they cover just 0.1 percent of the ocean floor.

Microplastics come from two main places: pieces made small on purpose, like microbeads, and bits that break off bigger plastic. They have been found in oceans around the globe and are eaten by small animals at the base of the food web. Sea animals and birds can mistake plastic for food.

As of November 2026, scientists know of about 226,000 ocean species, but they estimate the ocean holds 700,000 to 1 million species (not counting most microbes), so most are still undescribed.

Words to know

• subduction: sliding under
• trough: wave's low point
• crest: wave's high point
• seagrasses: ocean flowering plants
• microplastics: tiny plastic bits