Overview: A city known for its soy sauce production, Choshi lies 100km east of Tokyo, in Chiba Prefecture. Soy sauce production began during Japan's Edo era, although it was widely used on sushi and tempura before then. But why Choshi? For Ground Detective Simon Wallis, the hints this time are "hard coastal rock" and "flatlands," which are said to be particularly suited to soy sauce production. But how could geology make such a difference? The Ground Detective is on the case!