Problem pond: Help with construction, please

The previous owners of our house built an absolutely stunning example
of a horrible pond - it would lose around 40 gallons a day to leaks. We
decided to destroy and replace it, and we would have succeeded too -
had it not been for the fact that the bottom of the pond was built with
reinforced concrete and a clay bottom. The underliner was about as thin
as a trashbag - it would flake off in a high wind.

How can I remove this reinforced concrete, and excavate the clay
underneath the pond? I've tried a 12 pound sledgehammer, crowbar, and a
large pickaxe. All I've succeeded in doing is pulverising it.

The only thing that seems to cut into the clay is the pickaxe, so I'm
in the dark as to how I would remove that as well. I'll update this
topic with pictures of my progress if the need arises.