XP-61 is the name given to the deadly toxin made by the Pacifican Scientists. It was initially developed to be contained by the Erebus Dreadnought then dispersed into the Atlantic State's cities to kill the citizens of the East. But their plan failed miserably with the intervention of Jet Brody, one of the Atlantic Alliance's best soldiers. The substance is both posionous and acidic. It is found to be flowing in 'streams' at the development facilities where it, and the rebellious Creepers were created.