Today’s Wonder of the Day was inspired by Baylee. Baylee Wonders, “Why do worms live in the ground?” Thanks for WONDERing with us, Baylee!

Have you ever watched a worm move? They don't move very fast, do they? Unless they're in the belly of a bird — a place no worm wants to be! — they don't fly south for the winter. It's also highly unlikely that they drive or take the bus. So where do they go in the winter?

If you're a regular visitor of Wonderopolis, you probably already know that earthworms are good for gardens. But gardens don't grow year-round. So what do earthworms do with their time in the winter when they're not gardening?

The answers to these questions depend in part on what type of worm you're talking about. There are a lot of different types of worms. Would you believe there are over 4,400 different species of worms around the world? It's true!

Of those thousands of types of worms, over 2,700 of them are species of earthworms. Only a fraction of those can be found in the United States. Still, American gardens are called home by at least 30 different types of earthworms.

Some earthworms choose to live their whole lives in the ground's upper layer of soil and leaves. These earthworms never burrow deep into the soil, so cold winter temperatures kill them.

To keep their species alive, however, they lay eggs in tiny sacks that protect the eggs from freezing or drying out during the winter. In the spring, the eggs hatch and a whole new group of worms is born to repeat the life cycle.

Other earthworms, such as the night crawlers often used as fish bait, live close to the surface in warm weather and down deep in cold weather. When winter hits, these worms burrow down below the frost line.

The frost line — sometimes called frost depth or freezing depth — is the depth at which the groundwater in soil usually freezes in winter. The frost line varies based upon climatic conditions. It can usually be estimated based upon latitude. The farther north you travel, the deeper the frost line.

In the United States, the frost line ranges from zero feet (in warm areas like Florida) to six feet or more (in cold areas like Alaska). To survive freezing cold temperatures, worms must burrow to an area below the frost line wherever they live.

Night crawlers, for example, can burrow to depths of six feet or more. When they burrow down below the frost line, they nest in small chambers at the bottom of the tunnels they dig. Since worms can't breathe if their skin dries out, they coat the sides of their nesting chambers with a slimy mucus to keep them moist through the winter.

Once worms burrow far enough underground, the soil stays at a fairly constant temperature that keeps the worms warm through the winter. This warmer soil down deep acts like a warm blanket and insulates the worms from the cold.

Night crawlers don't really hibernate like some animals do in the winter. If there is a warm spell during the winter, night crawlers will occasionally come back up to the surface for a while until it gets cold again. When spring returns, worms make their way back to the surface.

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Tomorrow’s Wonder of the Day is sure to make you smile!