What adaptations to the lifestyle have underground animals (mole) more valuable species of mammals?

The fauna of the subterranean environment is not as diverse as the animal world of the aquatic or terrestrial-air environment. But a large number of species still live there. Animals have special devices for moving. These can be limbs or a special body shape. Most of these animals do not have visual organs or they are poorly developed. But the main adaptation of underworld animals is not some parts of the body. First of all, this is adaptability to living in a state of constant hypoxia, since there is little oxygen underground.