On July 1st I discovered a puddle of liquid mercury in our house. It was the evening before the holiday weekend and I was preparing to do some more demolition for the bathroom I've been working on. I was putting down plastic to make cleanup easier, and when I got to the corner there was a dusy little puddle. That seemed peculiar, so I poked it. It wiggled, and I knew exactly what liquid moves like that. Completely dumbfounded, I went downstairs to wash my hands.
A bit of looking online, it looked like I was dealing with a spill of about a tablespoon, and following their advice I called the fire department. And posted on facebook. Some firefighters came out, poked at it with a stick, and said "yup, that's mercury". They called out their chief, who also poked at it and agreed it was mercury.
A truck from the DEP showed up with hazmat suits and a mercury vacuum, and some firefighters suited up and started seeing if they could clean up the spill. After a couple hours they realized there was too much, and a civil engineer with the DEP told me that we would need to hire hazmat cleanup contractors to finish the work. A representative from the Somerville board of health officially closed the house, and David and I went back to Medford for the night.
So where did this mercury come from? The largest concentration was immediately below a pipe in the attic that was probably for an expansion tank for the original heating system. The house was built with hot-water radiators, but this was back before pumps were cheap enough to put in houses so it was a gravity based system. Hot water rises relative to cold water. This works better at hotter temperatures but you can only get water so hot before it boils. Unless you pressurize it! So people would use a mercury seal to put pressure on the water, allowing the system to run hotter without boiling. This is a "heat generator", and was the first product of the Honeywell corporation.
In the 1970s, when this system was converted from gravity-circulation to pumping, they apparently removed the heat generator, and spilled the mercury all over the floor. It ran under the floor, along the subfloor, seeped through the ceiling below, and got absolutely everywhere. And there it sat, mostly undisturbed, until I found it when doing demolition for the bathroom.