Tabun.

About Tabun

Tabun is an extremely toxic compound and nerve agent chemical weapon of the organophosphate family. It is not present in nature. At room temperature, the pure compound is a clear and viscous liquid. However, impurities imparted during its manufacture are almost always present, turning it yellow or brown. Exposed to environs, it slowly evaporates into the atmosphere, with the vapor having a slight fruity or almond-like odor. As the compound has a much higher molecular mass than air, tabun gas tends to accumulate in low-lying areas.