After it rains, people often step outside and think, "It smells like worms."
The smell is more likely caused by soil bacteria released after heavy downpours, but perhaps we think it smells like worms because we see worms more often after it rains.
Why is it that we see worms after it rains?
First, let's talk about different types of earthworms. There are three types of earthworms in Iowa --- anecic, endogeic and epigeic.
Anecic earthworms are capable of burrowing up to 6 feet below the surface, building permanent burrows in mineral layers of the soil. They drag organic material from the surface into the burrow for food. Examples of anecic earthworms are nightcrawlers, bait worms and dew worms.
Epigeic worms live on the soil surface and don't form permanent burrows. These worms, such as red worms, manure worms, red wigglers and compost worms, feed on decaying organic matter.
Endogeic earthworms build non-permanent burrows in the upper mineral layer of the the soil and feed on organic material. They live in the soil but are usually not noticed until after a heavy rain, when they come to the surface.
There is speculation as to why worms come to the surface when it rains.
The reasoning for a while was that worm burrows would fill with water, and because oxygen is not diffused as well through water as through air that the worms would come up to the surface to prevent drowning. However, worms can survive for several days fully submerged in water, so that theory was debunked.
Scientists have thought perhaps it is to help with travel, because it is easier to move great distances on the surface than through the earth. They normally wouldn't have enough moisture on the surface, because they have to be damp in order to absorb oxygen through their skin. But after the rain, they have enough surface moisture to migrate.
Another theory is that the vibration of the rain on the surface sounds like a predator, such as a mole, so the earthworms are trying to get out of the path of their predator. Humans create vibrations when searching for bait earthworms, and even birds will drum their feed to create vibrations and bring worms to the surface.
Even if only a small percentage of earthworms are tricked by rain's vibrations, there is as many as one million worms in an acre of soil, so even a small percentage can seem like a lot!