Crawl space
The shallow unfinished area under a home with no full basement.
A crawl space is the low gap between the ground and the first floor of homes built without a basement or slab. It houses plumbing, ducts, and wiring, and it's a major source of moisture problems: standing water, humidity, and poor ventilation here lead to mold, rot, and pests upstairs. Keep it dry, check the vapor barrier, and look for water after heavy rain.