Water in the whole crawlspace
Some homes will only have little puddles or "trenches" around the edges on the crawlspace. Others, like this one, will have several inches of water throughout the entire crawlspace.
Standing water leads to smells, rot, and mold
When there is this much water under a home, there will most likely be a lot of wood rot and mold. The smells from things like that are just awful.
Damaged air ducts and insulation
When the water evaporates into the air it creates higher humidity. That moisture will destroy the insulation and duct work. Rust, mold, and mildew will be everywhere.
What kind of allergens are coming in trough the air ducts?
Having air ducts directly above the exposed stagnant water is no different than living on top of a swamp. Often times there will be mosquitoes and other bugs and critters living in the crawlspace. This is a perfect breeding ground for mold, mildew, fungus, and insect allergens.
Damage to pipes and other equipment in the crawlspace
Many pipes are held in place by metal hangers. We build houses with nail and use metal fittings for several things. The fiberglass insulation gets held in place with metal pins. All of these things can rust and create a dangerous environment. And what about the electrical wires in the crawlspace? Imagine standing in water and touching an exposed wire, that is very bad.