An ocean is one of the huge bodies of salt water that cover most of Earth's surface. Unlike lakes or rivers, oceans are vast, deep, and salty, stretching for thousands of miles. Earth has five oceans: the Pacific (the largest), Atlantic, Indian, Southern, and Arctic. Together, they hold about 97% of all the water on our planet.

Oceans are dynamic systems full of life, from tiny plankton to massive blue whales. Ocean currents act like rivers within the sea, moving warm and cold water around the globe and affecting weather patterns everywhere. The ocean floor has mountains, valleys, and trenches deeper than Mount Everest is tall.

For thousands of years, oceans connected civilizations through trade and exploration, though crossing them was dangerous and took months. Today, ships still carry most of the world's cargo across ocean routes, and scientists continue discovering new species in the ocean's depths.

When someone says something is “just a drop in the ocean,” they mean it's a tiny amount compared to something enormous. The phrase captures how incredibly vast oceans really are.