Fire and flood are things to protect your collection from, and so, apparently, are sinkholes: http://www.cbsnews.com/news/sinkhole-swallow-eight-cars-in-national-corvette-museum-in-kentucky/

As a museum volunteer myself, I know this incident must be very discouraging to the workforce at the Corvette Museum. Hopefully they'll be able to overcome this problem.

While unfortunate, it isn't that surprisng. That area of Kentucky is known for its karst geography and many small, medium and large cave systems that are carved out of the limestone.

Better a few cars than it opening up 15 feet to the left and taking out the support column holding up the roof.

I would broaden that out by saying environmental conditions are always your enemy. Be it Karst topography there...
in the north, snow loads...
in the midwest, tornadoes...
in low lying areas, flooding...
in California, wildfire...
yeah bet you thought I was gonna say 'earthquakes' but that's actually a case in point: California's building codes have earthquakes beat, the threat is getting blindsided by the other stuff. I worry for earthquakes in the zones that don't expect it.

And remember Fukushima, the ultimate example of getting blindsided: they had reactors designed with "defense in depth" so it can still be cooled despite earthquake damage, and then they didn't have any, because their earthquake prep was so good... at the expense of other kinds of prep, which ended up nailing them much worse.

So, what are you not expecting? (that is foreseeable. Obviously you can't foresee a meteor.)

You left out the catastrophic possibility that I think is forseeable, if you have buildings that have gas heat. That is the possibility of a gas leak and subsequent explosion. The solution is to resist the "edifice complex" and have adequately seperated buildings for functions that require heated buildings, such as offices, visitors facilities and archives, and rolling stock preservation & display buildings, which do not require heat if adequately insulated.