A quick glance at the roof indicates it is far newer than the original one. I wonder if the original roof was, perhaps sod? That would allow for cooler inside temperatures than this chipboard and shingle one. I would be careful and regularly test any water seeping in as this is shallow enough to allow seepage of surface water. Any animals around the lawn could be providing bacteria that would not be helpful. Placing closed cell foam on the outside of the walls would be much less picturesque, but good to keep out surface seepage with associated fungi, bacteria, etc. and maintaining temperatures more in a continuous range. The zone indicates much of NC and the upland region of SC, North Georgia (outside taller mountainous areas) and much of north Alabama. Current soil temps at 8" down are showing temps in the mid-to upper 70's. I would not suggest this would work as a decent tornado shelter. That roof is not likely structurally attached to the walls and even then, the cement on what appears to be limestone looks quite old and showing its age. Does the home have a basement? I would guess not or it would be wetter than this. In the areas most commonly identified as zone 8a, tornadoes have a bad habit of hiding until they pounce. It is not a good idea to have a shelter outside the home in such areas, too much exposed travel in potentially deadly conditions.
The vegetation beyond the springhouse in the first picture with the home appears to be heading down into a channel. That would indicate potential for flooding and if so, the remnants of a flood in a building like this are less than desirable.
It definately looks cool and I really wish it could be easily modified to be worth the upkeep. Perhaps there are other homes with such structures in the area? If so, ask the owners if they know what they were used for. On meditating on the photo of the interior, what if the roof was modified to a greenhouse and then used for starting plants in the spring. I could really see that as a use since drought would be past the plant starting portion of the season, but the chances of frost past February inside would be minimal. Any open water on the floor would help to maintain a frost-free growing area with favorable humidity. Just a wild thought.