u/Giroy

I'm removing a walkway. After getting about 80% done, I stumbled upon something under the walkway. It almost look alike another old walkway, but seems much harder and more solid than the current walkway. It also sounds like it may be hollow, but hard to tell. Any ideas what it could be and should I be concerned about damaging it?

https://imgur.com/a/gFNKD3s