Excavating Under a Concrete Foundation Slab?
30' x 12' is significant. Do not use this slab as a ceiling for anything accessible by humans and do not excavate under it, without having a professional contractor or engineer assess it first. If it was not engineered for that purpose it is not fit for that purpose and it is not worth the risk to assume fitness.

It should not be a big deal to support it from below with posts and beams.