Well if a kid in a garage can make a partial nuclear reactor... Maybe make an EMP?
Edit: not a true reactor, but he was able to make some parts for it and supposedly got worried about the the amount radioactive material he had and began dismantling it but got caught with radioactive material.





Those stairs look quite old. Could be a food storage area like someone else mentioned. And it makes sense because it's well built and hidden well. Probably to keep passers by and animals out.