Microfauna are small animals and unicellular organisms visible only under a microscope. Usually, they are defined as creatures smaller than 0.1 mm (100 microns) in size, with mesofauna as organisms between 0.1 mm and 2 mm in size, though definitions may vary.
In the soil, microfauna can be found in large numbers — generally several thousand per gram. Anyone can take a bit of wet soil, put it under a microscope, and find these organisms. Some of the most common and important examples are protozoa (unicellular eukaryotes), mites (among the most diverse and successful of all animals), springtails (related to insects), nematodes (transparent wormlike creatures), rotifers (named for their wheel-like ciliated mouthparts), and tardigrades, also known as "water bears," one of the hardiest organisms in nature. Microfauna can be found worldwide, wherever there is wet soil, and some other places as well. Springtails have been found in the McMurdo Dry Valleys of Antarctica, one of the coldest and driest places on Earth.
Microfauna are accompanied by microflora, which includes algae, bacteria, fungi and yeasts, capable of digesting just about any organic substance, and some inorganic substances, such as TNT and synthetic rubber.
Larger animals also found in the soil are called mesofauna, such as earthworms, arthropods, and large nematodes, and the macrofauna, which includes burrowing mammals like moles and rabbits. The microfauna are the least understood of soil life, due to their small size and great diversity. Many are members of the so-called "cryptozoa," animals that remain undescribed by science. Out of the estimated 10-20 million animal species in the world, only 1.8 million have been given scientific names, and many of the remaining millions likely belong to this group, many of them in the tropics.
Microfauna live in tiny pores in between grains of soil, and many are aquatic. Some are sessile, meaning they attach to a substrate their entire lives and never move. These animals fertilize their mates by releasing ciliated sperm, while they themselves stay put. They can be found on the human body as well, and the average human home has thousands of mites that make a living by digesting dead skin cells.