I was reading about a nutrino detector site in Japan. They use ultra pure water. Ocassionally, guys in a raft have to float out into a tank to do maintenance on things. One worker dropped a tool in the water a few years back.
A scheduled cleaning of the tank required it to be drained. They found that the tool that had dropped previously, had completely dissolved. Apparently, even dipping your hand in the water for any length of time is considered unwise.
Water: the universal solvent.
Then what do they keep it in?