Magnesium phosphide, Mg3P2, is a category I toxin used in fumigants. The substance quickly reacts with water (rain!) and stomach acids of rodents (or humans) to produce phosphine. Phosphine an an odorless, colorless, and flammable toxic gas that acts similar to the mustard gases used in Word War I. Fortunately, the reactants of a spill will spread into the atmosphere and yield non-toxic substances. Therefore, avoid any areas with a spill until the reaction goes toward completion and the gas has dissipated.
As a tangent, even our beloved Antoine Lavoisier worked with the gas to determine its structure.
