A biome is a large naturally occurring community of flora and fauna occupying a major habitat. There are several main types of biomes: aquatic, desert, forest, grassland, and tundra. Aquatic biomes encompass freshwater and marine environments, and make up 75% of the Earth's surface. Desert biomes can be hot, dry, or cold, and contain specialized vegetation and animals adapted to their conditions. Forests cover one-third of the planet's land and store large amounts of carbon. Grasslands expanded during warmer periods in the past and include savannas and temperate prairies. The tundra biome is the coldest and has short growing seasons with poor soil quality.