Big Ben is an iconic clock tower in London that everyone knows about. It's not just a regular clock, though - it's got this massive hour bell that chimes every hour, and smaller bells that chime every 15 minutes. It stands tall at the northern end of the Houses of Parliament and the clock is super accurate, thanks to fancy gravity escapement technology designed back in 1851! Fun fact: Big Ben used to be called St. Stephen's Tower, but they changed it to Elizabeth Tower in 2012 for the Queen's Diamond Jubilee.