Board Thread:General Nerf discussion/@comment-3362895-20171027012135/@comment-27306930-20171027014150

Okay then, if I had to guess, someone left old Heavy Duty carbon batteries in the tray way too long. Unlike alkalines that leak white crystaline salt, carbon batteries leak a black mixture of graphite and manganese. It's also salty, and corrosive, but black and gooky rather than white and fluffy like alkalines.