What is song London Bridge about?

"London Bridge Is Falling Down" is a nursery rhyme and singing game from at least the 17th century. The song portrays the collapse of London Bridge and has several characters trying to repair it, including a cat and a mouse, though they prove incapable of the task. The song ends with the fall of the bridge and with it the collapse of all of London.

However, there is no evidence of the bridge ever having collapsed, and the rhyme refers to the upkeep and maintenance carried out on London Bridge over many centuries.