large leaky hole in concrete stairwell from splitlevel to sub_basement.

Howdy guys, I think I have a real odd one here! my house was a regular 70's split level, which was added too with an ground floor extension with a full basement. there has been water leaking out from beneath the stairs connecting the sub basement to the split level basement for as long as we've been in the house. The previous owners carved a channel in the concrete and and that directs the the water into a sump pump. Now because of higher than should be radon levels and because I thought it might be a good idea to actually figure out what was leaking I cut a hole in the stairs and took a look at our leak. What I found was a two foot wide by two and a half foot high hole where concrete should be but isn't. The concrete just seems to have melted away or was never there at all and the exposed bedrock underthe split level basement is right there to be poked out and leak water out of. Now my question is can I just construct a form and fill that in with some kind of concrete and adhesive? this is a really weird situation its not like I can dig out from under my splitlevel basement to put a waterproof barrier on the exterior side of this concrete stairwell it's literally under my house! Any ideas would be greatly appreciated. Thanks