Metal pipes appear as grey, cylindrical pipes made of metal which is sometimes seen bending. They often times are found making platforms and walls, with several pipes stacked on top to make this structure.
Metal pipes are first introduced in Toxic II. They are found in certain parts of the second robot factory making up platforms and walls. They are found twisting and leading to places the player (often times) cannot reach.
They cannot be destroyed by a bomb, as they are made of metal and most metal objects cannot be destroyed (with the exception of robots in the factory). Metal pipes are sometimes seen leading off the screen, but in some levels they lead into stone platforms, where the pipe presumably continues to another destination.
Sometimes they are pouring (possibly disposing) liquid into a pool of acid that will not overflow, nor will the pipe pouring it stop pouring liquid. Jumping into the trail of poured liquid will cause Toxic to lose one fourth of their health, but still have the same momentum they had when jumping. Doing this is futile, as there is never anything on the opposite side of poured liquid.