It is the formula for hydrogen sulfide, a poisonous gas that can be very harmful to tissue in the body if it is contacted in any way. It can also be found in a liquid or gel-like form.
While it was in compressed form of gel, the poison was fast moving, and was capable of killing victims by completely covering them, and causing immense burns and pain upon mere contact. The liquid form of H2S is also highly volatile when set aflame directly.
This gel-like form of the gas was made more dangerous by the Devil Fruit Caesar Clown "fed" it, giving the mass of gas life and sentience. The resulting "creature", Smiley, could move and attack opponents on its own, and even shoot parts of itself across great distances to reach places it couldn't otherwise. When Caesar Clown fed Smiley a special candy-shaped "bait", Smiley turned into an even deadlier gaseous form.
Four years ago, Caesar Clown triggered H2S gas to be leaked out on Punk Hazard as retaliation for being fired. This devastated the island's ecosystem, killing many forms of life on it, but he remembers the fact that some of them survived. Caesar then accumulated the gas and compressed it, thus creating his artificial creature and weapon of mass destruction named Smiley. It is one of the chemical compounds that Caesar uses as a poisonous gas.
When Caesar decided to debut his newest gas weapon to brokers across the world, he lured Smiley with a piece of "bait" in the shape of a giant candy, which, when devoured, had Smiley regress back into his original gas form, with the bait's ingredients converting the H2S gas compound into an even deadlier form, decimating many of Caesar's centaur subordinates.
- One Piece Manga and Anime — Vol. 68 Chapter 675 and Episode 601, Caesar explains to brokers about the gas H2S.
- One Piece Manga and Anime — Vol. 68 Chapter 668 and Episode 594, centaurs die after being run over by Smiley's H2S body.
- One Piece Manga and Anime — Vol. 68 Chapter 670 and Episode 596, Marines suffer severe burns after touching Smiley's pieces, and the slime explodes when set ablaze.
- Hydrogen sulfide — Wikipedia article about the real chemical compound.