The 1854 London cholera outbreak prompted Dr John Snow’s famous “experiment…on the grandest scale”, widely cited as one of the earliest known natural experiments. By comparing cholera deaths among households that received a supply of contaminated water with those receiving a cleaner supply, Snow sought to test his theory (against prevailing wisdom) that cholera is a waterborne disease. But, what makes this 19th century study particularly remarkable is that it is an early example of research transparency.