Wood Rot
When wood is broken down by mold spores and fungi it is easily broken by just touching it.
Sinking or Sagging floors
The reason that you have sagging floors could be because of what is going on under your crawl space.
Mold
Mold on floor joist are caused by too much humidity in the crawl space. Mold Spores could travel into your home and cause issues for those with allergies and asthma.
More Rot
Wood is easily broken and crumbles in your hand, imaging walking on this floor! Eventually you could just fall through!
More Mold
Mold can look a variety of ways. Always consult with a professional like DryZone if you suspect you have a mold issue.