We owned a place here in central Indiana that had a small reinforced concrete shelter underground below an outbuilding. It may have been for tornados also because the house didn’t have a basement. Not really sure about the details because the original owners were dead by the time we bought it.
It was damp in the driest of times and full of water in the spring. I had it filled in when we razed the outbuilding to build a new one.
You created a future archeological find!