Crawlspace
Our house sits on a crawlspace. One plumber commented that it's a "nice huge crawlspace big enough that you can crawl around on hands and knees".
My expectations for a crawlspace is that it should be dry. It's under the house, protected from rain. Water shouldn't be going there. Reality isn't meeting with my expectations. Every time it rains our crawlspace becomes a mud pit under the house.
Water evaporates into the crawlspace air. Settles on woodwork, condenses on galvanized pipes. Generally doing the nasty corrosive things that unchecked moisture does. The under-the house moisture then wafts its way up into our living space. It's condensing on the single pane windows, dripping onto the wooden window sills and rotting the wood. We have spots of green and white mold growing on the newly-installed-by-the-previous-owner unsealed baseboard. We also have spots of black mold growing on the plaster of our un-insulated exterior walls.
Uncontrolled moisture is evil.