The cathodic protection electrochemical reaction generates gas as part of the reaction process. This also liberates electrons to allow current to distribute through the electrolyte. In most environments that gas is able to diffuse or discharge somewhere. However, in those rare cases where the generated gas cannot migrate away from the anode surface, the gas can actually block the flow of electrons and choke the cathodic protection reaction. This is more common within deep anode systems where a hole is bored from the surface down into the earth, and the surrounding environment around the bore hole may not be very permeable thus trapping gases. Most deep anode systems use a vent pipe to allow gasses to vent away to prevent gas blockage.