Remember that scene in The Abyss, before you fell asleep, when Ed Harris was put in a diving suit that was filled with pink goo that he then breathed? It turns out James Cameron wasn't blowing pink goo-laced smoke up our ass; that stuff really exists. Perfluorocarbons are fluids that contain shitloads of oxygen, making it possible to breathe liquid. They originally tested it back in the 60s on mice, with a certain degree of success... sort of.
The mice ended up dying after being submerged in it for a few hours, possibly due to the gut wrenching horror of drowning, but not dying, while trying to scream in their tiny mouse voices. Rather than manning up to the fact that breathing liquid destroyed the mice's diaphragms, the scientists blamed the deaths on impurities in the liquid (most likely horrified mouse shit).
What the Hell is it Used For?
Aside from slow, tortuous rodent murder, perfluorocarbons are used for ultrasounds, and even artificial blood. But before you go out and fill your pool with some for a leisurely four-hour swim at the bottom, be aware they are also awful pollutants. The worst offenders have a half life of 50,000 years and warm the atmosphere 6,500 times more effectively than carbon dioxide, and God knows how many cow farts that would be. Supervillains of the world, we hope you are taking notes.