A nurse at a cancer ward in Kansas noticed that many of the patients with the same rare cancer were originally from Buffalo, New York.
So she got a map and some pins, and stuck a pin on each address of each afflicted patient.
Turned out that all of the addresses were located very close to an artificial stream called the Love Canal. Looking in more detail, she found that the closer geographically their addresses were to Love canal, the more severe were their symptoms, and their life expectancies was shorter.
Suspicions were raised further, and the polluted Love canal was found to have a high concentration of a cancer causing chemical. The association between addresses near Buffalo and a rare form of cancer found in a Kansas cancer ward were no longer considered coincidences.
This study was published in the journal, Science.