Listen to part of a lecture in a biology class.

Many animals live underground, in the soil—not just little animals like worms, but also bigger animals, like mammals. Living underground has advantages—it protects animals from above-ground predators. However, the underground environment also presents challenges… and animals that live underground have developed physical adaptations to deal with them. One challenge is, well, simply how to move underground, through the dirt. Another challenge for the animal is to protect vulnerable parts of its body from the environment that it's moving through.

Now, moving through soil is not like moving through air or water, because soil—earth—is thick and dense. So animals that live underground have evolved physical features that help them move through dirt efficiently. For example, the mole, a small, furry mammal, has really wide, super-strong front feet with big claws. The mole's feet act like, uh, like shovels—so it can dig through the dirt. The claws cut into the dirt, loosen it up, and then, once the dirt is loosened up, the broad feet throw the dirt behind the mole as it moves forward. These shovel-like front feet allow a mole to dig its way through the dirt astonishingly quickly.

But even for an animal that can move efficiently through the dirt, living underground can still be problematic, because it's easy for particles to get caught in sensitive parts of the animal's body—like, for mammals, in their eyes. So underground animals have developed adaptations to prevent this. Again, let's take the example of the mole. To begin with, moles have tiny eyes. And these eyes are covered with a thin skin, a protective membrane that's actually got hair on it. These hairs protect the mole's eyes from dirt particles. So as the mole goes digging through the dirt, with its head pushed forward, the dirt particles come into contact with the hairy membrane covering the mole's tiny eyes, and the particles just slide by, don't get caught in the mole's eyes. So the eyes—the mole's sensitive parts—are protected.