This is one of those things we see which people "custom build a house around" without fully taking in all the effects of it.
Sure, they may have thought "That would sure look neat *in* the house, Mr. Builder, build the house right here, and leave that rock - I want it as a focal point in the living room.", but what about the next owner - I am sure it *adds value* to some, while to others it is a negative aspect and keeps them from buying it.
Maybe it should be called the dying room?
If radon is that much of a problem there, then the radon level would likely be high anyway, whether or not the rock was *in* the living room or *under* the living room, unless a radon mitigation system was installed under the slab.