Pretty much it’s common problem with this type of duct work. It’s was made huge mistake sometimes in 70 ties. They just laid duct pipes on the bare ground and poured concrete over. Now all those galvanized pipes rotted and there is nothing you can do to fix them. So the only solution is to abandon all those ducts and pour concrete in them. Then you need to switch your ducts from downflow to up flow and run ducts in the attic. If it’s a 2 story house then it’s even more complicated. Will be expensive anyway. You can look up on you tube or Holmes on homes Canadian show had same problem and he did what I just said. Pretty much it’s bad and not easy fix.