 Fictional subterrenes are often depicted as cylindrical in shape with conical drill heads at one or both ends, sometimes with some kind of tank-tread for propulsion, and described either as leaving an empty tunnel behind it, or as filling the space behind it with mining debris such as the Thunderbirds Mole.The plausibility of such machines has declined with the advent of the real-world tunnel boring machines, which demonstrate the reality of the boring task.Tunnel boring machine themselves are not usually considered to be subterrenes, possibly because they lack the secondary attributes - mobility and independence - that are normally applied to vehicles.