Anyone who's ever taken a taxi in London must have wondered how the city's cabbies know how to reach even the most obscure destination without a street map or a hint from their passengers. A new study indicates cabbies are working their brains so hard they become enlarged in the zone associated with navigation -- the rear hippocampus. It seemed that the drivers' brains adapted to help them store a detailed mental map of the city, shrinking in one area to allow growth in another, according to the study, published Tuesday in the journal Proceedings of the National Academy of Sciences.