The first problem is the owner, FannieMae never lived in the home and may be exempt from completing the Seller's disclosure. They likely didn't even know about it. Based on this new information, if you really want out of the deal, you should start by discussing it with your broker. If they don't see a clear contractual path, then have the contract reviewed by an attorney and explain what happened. This discovery after the inspection will likely complicate things, but there still may be a provision for you to terminate the deal at no cost. The very worst case scenario is that you lose your earnest money, but see if you can find away to avoid that.
Safety might be a concern. Is there some reason it can't just be capped off to keep someone from getting in? Does it need to be restored?
I would think that more investigation would be needed if you are not sure how it affects your decision.
If you love the house and it can be remedied then it may not be an issue.
You questions may be regarding safety and resale. Safety would need further investigation, resale not many people may want a bomb shelter.
This should be discussed with your agent and then addressed how you see fit.
Have an amazing day!
If you are using the standard Inspection Addendum from NWMLS, no reason is required if you are within your timeframe. Since you didn't expect the shelter, does the current condition really bother you? I would want to know what risks the shelter may pose if ignored or to find out what it may take to bring it up to shape. One other option you have with form 35, the inspection addendum is to request additional time for further inspections. Depending on how the form was completed you may have up to 5 days to investigate further.
Let us know how you proceed on this one.