Lesser known of the carbon oxides. Foul-smelling and lachrymatory gas (evidently so smelly it makes you cry) that becomes an unstable solid polymer at room temperature. C3O2 forms polymers that are red in colour.
It occurs naturally only under specific conditions. Photochemical processes can break down atmospheric carbon dioxide and subsequently atomic carbon combines with carbon monoxide. This is how it's believed to form on Mars, accounting for some of the planet's red colour. It's also thought to be present in Halley's comet and possibly responsible for its low albedo.
Carbon suboxide splits into two carbon monoxide molecules and one carbon atom when exposed to 157nm ultraviolet light. Its molecular weight is 68.032. It's stable at -78°C so, though it has no industrial uses, it might be suitable for deep-frozen stink bombs.