Why can you see a lot of earthworms on the soil surface after a heavy rain?

The soil is not a solid substance, but a network of small channels and microscopic reservoirs through which air and water circulate. When it rains, water seeps into the soil, clogging up all the tubules and displacing air. Earthworms living in the soil have nothing to breathe, and therefore they crawl to the surface so as not to suffocate, and after a while, when the soil dries up, they return back.