Help! Buried patio.

I've gradually been renovating one garden since moving in, and was hoping today to plant some roses. However, there's what I can only assume is a concrete patio in the way! I knew there was some concrete to shift, and the sledgehammer had dealt with the first bits easily, but the rest send made of sterner stuff and it's making dents, but only that. Any ideas for the best way to shift this? It's also do thick that in wondering if it's something other than a patio, but I can't imagine what. Given its on a slow, I can only assume they made it super deep to act as a retaining wall too. In case it's not obvious from the photo, the concrete is at least 25cm deep - possibly more.
Advice welcome!
Advice welcome!
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