Heavy Ice

Ice floats on water because it is less dense, but ice of a special kind can be denser than normal water. "Heavy ice" is 10.6 percent denser than normal water because the ice is made from "heavy water". Heavy water, D₂O instead of H₂O, is water in which both hydrogen atoms have been replaced with deuterium, the isotope of hydrogen containing one proton and one neutron. Heavy water is indeed heavier than normal water (which contains a tiny amount of heavy water molecules naturally), and heavy-water ice will sink in normal water.