The mercury will sink if you break a thermometer underwater.

Mercury is an element, and although it's a liquid at room temperature, it is very heavy. Not only does it sink in water, but heavy solid objects, such as iron cannonballs, will actually float in a pool of the silvery metal.

Density Comparison

If you pour a quantity of water and mercury into a container, the water will immediately rise to the top and cover the heavier liquid. This happens because mercury's density is 13.5336 grams per cubic centimeter. This is more than 13 times the density of water, which is 1 gram per cubic centimeter. A unit of mercury therefore weighs over 13 times more than the same unit of water.

Mercury Thermometers

Mercury is the only metal that is a liquid at room temperature, and although it is a poor conductor of heat, it expands and contracts evenly in response to temperature changes. Moreover, because it is so heavy, it has a high boiling point. These qualities make it a suitable material for thermometers. Mercury is toxic, though, and mercury thermometers are gradually being replaced by alcohol-based or digital ones.