A dirt crawl space under a home is a very bad idea. The earth has very high humidity in the soil. This water vapor moves easily into the crawl space and upwards into the house environment. A damp environment is very unhealthy and destructive. Mold thrives and reproduces by producing airborne spores by the millions, and some molds are toxic.
Insects and critters of all kinds love damp environments and wet materials. Rot and decay occur in damp environments causing structural damage. The natural airflow in a house is from the bottom to top. Thus sucks moist air and everyhting in it up into the living areas of the home. Mold spores, odors, humidity and critters create a very unhealthy environment for people.
Many people are allergic to these things, and experience a host of symptoms and don't relize that their dirt crawl space is affecting their health. Dirt crawl spaces do not have to be flooded to be extremely unhealthy.