The soil is then excreted with mucus to form burrow walls. Castings, which are excreted wastes and dirt clumps, show up on the surface of the ground. They look like tiny bunches of grapes. You've probably seen them in wet moist places. If not, go out and try to find a bunch.
Castings help fertilize the ground.
That's worms in a nutshell, rather, in the ground. Here's how you can watch them make better soil right in front of your eyes. This might be a wooded area under a tree that is also in a low spot, or in a shaded part of a lawn after a good rain. Also look for the castings which mark the best spot to dig. Use a magnifying glass and observe the worm.