What is the stuff under my carpet?
 

So, my husband and I started living in our new apartment in Toronto. Last day, I pulled back the carpet in order to study the situation of the floor as we were planning to renovate the house and seek help . There was black mould all over the surface and I am confused on what this really is.
It is a mud-like substance, which is somewhat hard and can be scraped off and crumbles to dust. This stuff also is spread across the entire room.
Can anyone tell me what this crap is? I am really hoping this is some type of old glue and not a mould problem.

First floor or second. If the floor is susceptible to outside water, then it may be mud.

I am thinking very old adhesive.

Do you have access to a microscope. A low power (10-20 x) will reveal mold if it is that.

Mix some with water. Does it stink?

Steve

deteriorated carpet pad?