The water rights that govern how California’s water gets used are not the easiest things in the world to understand; working out how they came to be, and how they shaped Los Angeles, is an even trickier thing.

Tom Scott tells the story of one small part of it: the St Francis dam, a project finished in 1926, and which collapsed two years later. The flood killed 431 people and ended the career of engineer William Mulholland. But as Scott explains, the dam had an equally significant impact on how LA developed; if it hadn’t been for some bad calculations, one of North America’s biggest cities could be even more dominant.