Answer: Sulfur Hexafluoride

Many a child has entertained their friends at a birthday party by breathing in helium from a party balloon and speaking in a high-pitched voice until the helium in their lungs dissipated. The trick hinges on lowering the density of the air around the vocal cords and, in this case, by sucking down some low-density helium gas.

A less familiar, and more dangerous trick, is to repeat the process using a very dense gas–sulfur hexafluoride. Just like low-density helium displaces the air and creates a high-pitched voice, sulfur hexaflouride also displaces regular air but creates a very low-pitched voice thanks to its high-density. It is the high-density that makes the trick riskier than raising your pitch with helium–helium is less dense than air so it rises up your throat as you continue to breath and talk; sulfur hexafluoride sinks to the bottom of your lungs and many times you need to invert yourself to fully purge it out and restore your full lung capacity.