What is mercury. Mercury is a chemical element with the symbol Hg and atomic number 80 and is commonly known as quicksilver. The type of mercury that is in the water is called methylmercury and it is produced by the burning of waste containing inorganic mercury and from the burning of fossil fuels but mainly from coal. There are large amounts of concentrated methylmercury in fish that we eat. The bigger the fish the more methylmercury it has absorbed. The reason that there is a lot of methylmercury in fish is because of bioaccumulation. Bioaccumulation occurs when an organism absorbs a toxic substance at a greater rate than at which the substance is lost.