"Mercury in the environment but I can't remember if its the top cause."
It used to be used in very large scale electrolytic production of chlorine and sodium hydroxide from brine . The mercury acted as the cathode and dissolved the sodium formed. The mercury flowed in a loop and once away from the electrolytic cell the mercury/sodium amalgam allowed to react with water to generate sodium hydroxide. I think it was waste water from this process that caused the Minimata incident.
These days the electrolytic cell has a membrane barrier that can pass current so no mercury is involved. The membrane is neat and rather expensive called 'nafion'
In fact I've used it myself in an experimental electrochemical removal of a protecting group off a synthetic antibiotic years ago