Not medical advice: The gas that makes the human voice sound much deeper is sulfur hexafluoride. It is a non-toxic, invisible gas with the chemical formula SF6. It is often considered the anti-helium gas because helium is about six times lighter than air and sulfur hexafluoride is about six times heavier. It also makes the human voice deeper while helium makes it higher.
Those with a penchant for experimenting can use sulfur hexafluoride to float a "boat." To begin, pour sulfur hexafluoride into an aquarium or large beaker. Since it is heavier than air, it will sink to the bottom. You can float light objects on the invisible gas; i.e. a paper airplane or a boat made from aluminum foil. If you scoop up some of the sulfur hexafluoride and dump it into a foil boat, it will sink.
While SF6 is non toxic and generally safe, it should be used in a well ventilated environment. As a manufacturing by-product, SF6 is considered a greenhouse gas.
Caution: SF6 is described by Airgas, a company that sells it, as a substance that may cause damage to the upper respiratory tract.