The Web

The Web was an ill-understood substance that the Yeti used to ensnare central London. It could manifest itself like 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".

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.

Though it wasn't clear how the Yeti pushed vast quantities of the Web around the heart of London, they 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)