350 pounds (or 10 tons? Actual amount is unknown) of mercury
Here's a video of a guy flushing 240# of mercury down a toilet (NOT connected to a sewer or septic system), just to see if it was possible: https://www.youtube.com/watch?v=GvVaaZ21C44
It's not exactly difficult to find industrial users of mercury, and the mercury at Sunspot is in a sealed instrument. I see no reason why the presence of mercury there would be incentive for an "armed group" to attempt to assault and/or hold the observatory.
Obviously, I'd be sure of that before I handled it :)
See: mad hatters
https://sunspot.solar/about/ https://en.wikipedia.org/wiki/Liquid_mirror_telescope