They belong to a group of earthworms known as deep-burrowers. The deep-burrowers build large, vertical, permanent burrows that may reach as deep as 5 or 6 feet.
Nightcrawlers pull plant material down into their burrows which can be fed on later.
These middens may be used for protection and food reserves. The burrows can have a significant positive effect on soil by opening up channels for water and air to penetrate. Roots also like these channels due to the ease of root penetration and nutrients found in the casting material lining the burrow. Nightcrawlers actually help the soil but may make it difficult to mow. Getting rid of the middens will be difficult.