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Actually I have to do the same thing in my back yard, a tunnel into a 6" thick block wall. The cores of the cinder block have been filled with concrete. I'm going to bore a series of 1/2" diameter holes in the shape of the tunnel portal, somewhat inside the line I want. This is easy to do with a hammer drill and a carbide tipped masonary bit. It's easy to get this size drill long enough to go through. Then, you can use a small chisel to knock out the concrete between the holes. Wou will need to drill a number of holes, but you have much less risk of cracking the part of the concrete you want to stay. After that, a 3 or 4 inch diamond blade in a cutoff tool (I use air powered tools) to smooth the inside of the tunnel. I have asked a number of people that work with concrete, as well as my 2 landscapers who built the walls, this is the consensus. Regards, Greg
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