- You may be looking for the internet.
The Web (PROSE: Doctor Who and the Web of Fear) was the great intelligence ability to manifest itself as a mist or a fog, or it could appear as a cobwebby material. Still, some observers also called it a fungus. Prolonged exposure was certainly fatal to humans, though the means by which it killed were mysterious. Travers claimed that the mist aspect of it wasn't poisonous, but people who went into it "just didn't come out again". Staff Sergeant Arnold later made the point that the Web filled a deep vertical space, meaning that it wasn't possible to escape onto another line's tunnels, if one was trapped within the two-dimensional boundaries of the Circle line.
The Yeti, acting for the Great Intelligence, used it primarily to draw a ring around central London which followed the path of the Circle line. The British Army were initially helpless to do much more about it than to monitor the progress of the Web on a specially-made dynamic map of the line.
The Yeti were known to carry smaller quantities of it in Web-guns, which they could fire more or less like conventional pistols. (TV: The Web of Fear)