What's under a fireplace in a 1930's terraced home?

[fex10]

I've removed the fireplace from one of our rooms. Under it is a concrete base. I've lifted a floor board and under the floor is a brick wall which follows the concrete base.

Does this mean that the whole area within the wall is full of concrete? Photo below..

 

[stuart45]

It's the fender wall, and is usually filled with hardcore, and then 6 inches of concrete on top for the hearth.

 

[fex10]

Thank you!