I recently was under a house in southeast Missouri and found something I have yet to encounter in 5 years of inspecting. Any help on what this might be is appreciated. It looks like roots but it starts and ends on the individual joists.
Fungus , did you take a moisture reading?
I agree with Wayne, That is what it looks like.
Looks like varicose veins.
Agree with Wayne
Commonly called bootlaces
Most likely dry rot fungus, which in severe cases like yours is expensive to repair.