Student or Learner
What does 'underground' mean in the sentence 'Number seventeen was -of all things - underground'? (This sentence, without any context, is taken from COLLINS COBUILD ENGLISH USAGE, in the part of PUNCTUATION, dash.)
Is it the underground railway system?
It might be or it might not — it doesn't matter. The point of the sentence is to illustrate the use of the dash (as my sentence does).