Yes, everyone knows that ice floats - but you can actually make heavy ice that sinks to the bottom by using heavy water.
The key to the trick is heavy ice. Many terms shouldn't be taken literally—a red quark isn't red, a peanut is neither a pea nor a nut—but heavy water is exactly what it sounds like: water that weighs more than normal. This is possible because elements occur in several different forms, or isotopes, made up of atoms with the same number of protons and electrons (which determine their chemical properties) but a variable number of neutrons (which contribute weight but not much else).
Hydrogen atoms always have one proton and one electron, but only one in every 6,400 has a neutron that nearly doubles the atom¹s mass. Using a complex process called H2S, it¹s possible to isolate this heavy hydrogen, also known as deuterium (D), creating water that¹s about 10 percent heavier than normal.
Chemically, D2O—as it's written—is real water. Algae can grow and thrive in pure heavy water. Specially raised mice have contained as much as 25 percent heavy hydrogen; beyond that level, subtle biochemical reactions make the heavy mice sick. (Researchers used mice because they are small. Raising a heavy cow would be expensive.)