Two Ekaterina scientists were both trying to find this clue; Thomas Edison and Nikola Tesla. The two were bitter rivals, as they were both trying to find the clue. Tesla eventually found the clue and hid it in his lab in Croatia.
Mercury is an element that is extremely poisonous and can be lethal. It was originally used in thermometers but because of its harmful properties, isn't used in them anymore. Mercury, along with bromine, are the only two elements that are liquid at room temperature.