A rainforest is a forest where a lot of rain falls, and a tropical rainforest can get more than 2,000 millimeters (79 inches) of rain in a year. In the rain forest of Olympic National Park, precipitation ranges from 140 to 167 inches (12 to 14 feet) every year. The Amazon is the largest river on Earth by the volume of water it carries. As of November 2026, its flow is about 15 percent of all the fresh water that rivers discharge into the oceans.

You may have heard that rainforests must have rich soil. In fact, rainforest soil is poor in nutrients, heavy rain washes nutrients out of it, and rainforest soils lack minerals such as phosphorus, calcium and magnesium. Dead plant material in a rainforest breaks down fast, so there is very little leaf litter. In Olympic National Park's rain forest, many seedlings sprout on fallen, decaying trees because the ground is so densely covered, and massive trees there can take centuries to slowly decay back to the soil after they fall.

The largest unbroken stretch of rainforest is in the Amazon river basin of South America. The Amazon basin spans at least 6 million square kilometers (2.3 million square miles), nearly twice the size of India. As of November 2026, the rainforest covers about 80 percent of the basin. As of November 2026, the Amazon rainforest is home to around 40,000 plant species, nearly 1,300 bird species and 3,000 types of fish, and it has more than 430 species of mammals and 2.5 million different insects.

Some rainforest trees grow as tall as 60 meters (200 feet), and many rainforest trees have buttresses, large aerial extensions of their surface roots. A single hectare (2.5 acres) of tropical rainforest can hold hundreds of species of trees. As of November 2026, the Amazon has an estimated 400 billion trees. Trees pull water from the ground and release water vapor through their leaves, which creates moisture in the air above the forest. A single tree can transpire hundreds of liters of water in a day. Through its trees, the forest sends water back up into the air.

Words to know

• nutrients: food for plants
• buttresses: big root supports
• decay: break down
• basin: river's land area
• transpire: release water vapor