My son asked me how gasometers worked - in as much of when they are low, where does the 'building' go - does it concertina down or is there a dirty great big hole in the ground that it disappears into? I had to admit that I had no idea. Anyone care to enlighten us? Don't you just hate it when kids prove that their parents are fallible ? :o))
Further to kempies link, some do 'concertina' down (sort of like a giant hydraulic ram) and some go down in one piece; it just depends on the particular design.