Unknown for sure that they are there. The base does have the means and facilities to secure them. No one is going to walk in and take them.
Imagine a very, very deep hole in the ground inside the storage facility.
In a worst-case scenario you open the cover and lower the ordnance to the bottom.
Then you set off small charges that collapse the concrete walls and you have secured the ordnance from unfriendly hands for a considerable time...time to bring in the full force of the US military.
Of course, this is only a guess :-)