Which chemical element's symbol is Hg?

Mercury, from hydrargyrum - Greek for water-silver, which is exactly what it looks like. It is the only metal liquid at room temperature, and the reason is relativistic: its electrons move fast enough that their effective mass increases, contracting their orbits and holding them so tightly that mercury atoms barely bond to each other.

It was used for centuries in ways that now read as horrifying. Hat-makers treated felt with mercury compounds and developed tremors, paranoia and cognitive damage - mad as a hatter. It was in thermometers, barometers, dental fillings, laxatives and syphilis treatments, and Lincoln took mercury pills for melancholy and stopped because he thought they made him irritable.

The industrial disaster that changed regulation was Minamata in Japan, where a chemical plant discharged methylmercury into the bay from the 1930s. It concentrated up the food chain into fish, and the people who ate them suffered severe neurological damage, including children affected before birth whose mothers showed few symptoms. The company denied it for years and the courts took decades. The international treaty restricting mercury, agreed in 2013, is named the Minamata Convention.