Many folks, including residents of the city itself, don't know that London is built upon seven [at least] rivers, only one of which is still visible and identifiable as a river.
The hidden rivers that shape London's streets: Fleet, Westbourne, Tachbrook and Tyburn; all these and more gave the city its form
About twenty years ago, I recall a young couple bought a "fixer-upper" in London and couldn't figure out how their indoor cat kept managing to get soaking wet while indoors. It turns out, when the rains came, one of those dormant rivers came to life in their basement, with the waters receding dramatically and without evidence once the rains subsided.