Sulfur hexafluoride is one of the densest gases out there. It's so thick that it's possible to pour it into a container like a liquid and float a tinfoil boat on it. Check out the boat that floats on nothing.
Sulfur hexafluoride is made up of six fluorine atoms attached ringing a central sulfur atom. It's six times heavier than the air we breathe and a greenhouse gas. It's produced for the technology, medical, and construction industries, all because of one very useful property: it's inert. The gas reacts with pretty much nothing, making it useful for insulation, tracing gas leaks, and safely placing pressure on the human body.
People can even breathe it, as long as they breathe the right amount of it — which is practically nothing. Sulfur hexafluoride is often used to demonstrate the way the voice changes in timbre when people breathe different gases. Helium, another inert gas, is lighter than air and makes the voice squeaky. Sulfur hexafluoride makes the voice Satanically deep.
Sulfur hexafluoride voices aren't brought out during parties, and not just because sulfur-filled balloons would be boring. The heavy gas shoves aside air molecules and pools towards the bottom of containers, including lungs. It can't be easily pushed out of the lungs, and repeated breathing can smother a person even if they're in a room full of regular air. A sip of sulfur hexafluoride is all it's safe to take.
That particular property of sulfur hexafluoride, though is definitely the cause of some awesomeness as well. The gas, in air, will stay pooled in a container so well that it resembles a liquid. And it's so dense that a buoyant object that's light enough will float around on a sulfur hexafluoride sea. Take a look at the above tinfoil boat bobbing around in an invisible "liquid" until someone bails enough sulfur hexafluoride into the craft to sink it.