In the past two years, I have researched pox, croup, burns, mill fever. Historical wood paneling, cathedral architecture, farm outbuildings, whale blubber, color theory, wavelengths of colors. The geography of Dundee, Scotland, shipbuilding, sheep, udders, shearing. Vaudeville, theater, jute, Scottish food, cheese-making.
And that's just the tip of the iceberg. It's surprising what random information proves useful when you're writing.