First timer, here.
I recently purchased a home built in 1968. The home has been a rental property for several years and now I have decided to purchase it and began tearing out the cheap laminate flooring that covered the entire house.
Half of the house was originally carpeted and underneath that I found this material (4'x8' sheets) nailed to the subfloor. It is almost the consistency of dense ceiling tiles - very papery and fibrous. (See pictures).
I have already removed an entire room of this, and have more to do.