We don't often think about what's happening right under our feet, but there's plenty of life down there. One of the most populous — and probably the most recognizable — inhabitants in the soil down below is earthworms. These invertebrate creatures are neither insects nor arthropods, like most of our other subterranean dwellers. 

Earthworms are annelids, according to the University of Pennsylvania. In Latin, annelida means "little rings," and this is an apt description of worm anatomy. Worms are composed of many small segments fused together. Worms can have between 100 and 150 of these segments, and they can relax or contract to move through the soil.

As earthworms tunnel their way through the soil, they eat it, and in the process the decaying and decomposing organic matter in the soil. They also eat bacteria, fungi and other tiny organisms. The waste they leave in their wake, called casts, return nutrients to the soil.

Read on to learn even more about these totally tubular creatures.

They aren't native, but they are beneficial — mostly

While some earthworm species are native to parts of the United States, none are native to the upper Midwest. However, unlike some other non-native species, most earthworms are beneficial. Although they are not essential for a healthy soil habitat, they are usually an indicator that a soil system is healthy and well-functioning, according to the U.S. Department of Agriculture Natural Resources Conservation Service. 

Many of the non-native earthworms that have become commonplace in the United States have been here for centuries. Most are native to Asia and Europe and were likely transported inadvertently in ship ballast tanks or in the roots of plants being shipped to North America, according to Smithsonian Magazine.

While it's true that most earthworms enhance soil quality, this is not universal. A few worm species are actually detrimental to soil health. One example is the jumping worm, a large earthworm native to Asia. These destructive worms alter the soil structure, damage plant roots, deplete nutrients in the soil and diminish the water holding capacity of soil, according to the University of Illinois Extension. They were first found in Illinois in 2015, and today they have been identified in more than 35 counties, including Will County.

RELATED: INVASION OF THE JUMPING WORMS

There are thousands of different kinds

If you've seen one worm, you've seen them all, right? It might seem like that, but there is incredible diversity among earthworm species. The world is home to about 7,000 different kinds of earthworms, and they are divided into 23 families, according to the U.S. Department of Agriculture Natural Resources Conservation Service. The United States is home to more than 100 different worm species.

With so many species, there's a lot of variety among worms. Some are only an inch or two long, while others can be measured in feet. The longest ever recorded was 22 feet long when it was discovered in South Africa, according to the University of Illinois Extension. Some may come up above the ground from time to time, but others remain subterranean for their entire lives. 

RELATED: WHY DO WORMS COME TO THE SURFACE WHEN IT RAINS?

One thing that's universal among earthworms is the need for moisture. They will die if they dry out, so they need moist soil. When soil becomes too dry, they will typically burrow farther underground, according to the University of Michigan. For this reason, they are most abundant in rainy forested areas, although they can live in many habitats both on land and underwater, in the mucky, muddy soil. 

There's a whole lot of worms right under your feet

The number of worms living underground is almost unfathomable, but it's safe to say it's a whole lot. In most soils, earthworms are the most populous subterranean resident, according to the USDA. So just how many are down below? A single square yard of cropland in the U.S. can have between 50 and 300 worms living in it, and that's the low end of the population scale. A square yard of woodland or grassland can have 500 worms or more living in it. Think about that on a larger scale: There can be more than 1 million worms living in an acre of land, the Illinois Extension reports.

All those worms fall into three different groups based on what elevation of the soil they inhabit. Epigeic worms live in the soil at surface level. They are usually smaller species and cannot usually survive in soil with low organic content, the USDA reports. Below the surface soil, endogeic worms live in the upper level of soil. They mostly eat the organic matter in the soil. Down even further in the soil are the anecic worms, which include nightcrawlers. These worms eat litter in the surface soil that they pull into their burrows, and they typically live in networks of burrows that are mostly permanent. 

They have male and female reproductive organs

Earthworms are hermaphrodites, with all worms having both male and female reproductive organs, according to the University of Pennsylvania. However, worms usually need a mate to successfully reproduce. They typically mate when the ground is wet from recent rain. Two worms will line up next to each other, but facing opposite directions. The clitellum — a raised band on the front end of the worms — will secrete mucus so a slimy tube surrounds the worm. As they worms move forward out of their slimy tube, they pass over the female pore, picking up eggs in the process. As the worms continue moving, they pass the male pore, and sperm will enter to fertilize the eggs.

After mating, the slime tube becomes a cocoon to protect the eggs, which are placed in the soil, the University of Pennsylvania reports. It can take anywhere from a few weeks to several months for the eggs to hatch, depending on the species and the soil and weather conditions. When they do hatch, the worms are fully formed, fully functioning earthworms, but they are smaller than adult worms. They will continue to grow as they get older. 

They help prevent pollution and improve soil quality

Worms live in the ground beneath our feet, and all their activity down there has far-reaching effects, even helping prevent pollution, the USDA reports. When worms move through the soil, mixing it and shredding and leaving burrows as they go, they are improving the soil structure. When it rains, water is able to more quickly seep into the ground through the tunnels and loosened soil, which prevents water runoff and erosion. This also helps limit pollution in surface water by reducing runoff and increasing infiltration into the soil. Their underground activity also helps plants grow by creating channels for roots to grow.

Despite these positive benefits, worm burrows also could make it easier for pollutants like pesticides and nitrates to get into the groundwater, according to the USDA. How pollutants seep into and move through the soil is not an exact process, however, so it's not known whether the activity of worms in an area could have a negative effect on groundwater quality.