Summary: Scientist have been looking for a way to determine past weather patterns in the ocean, the same way as tree rings show weather patterns on land. Recently, it has been discovered that coral contains chemical signals of past water temperatures. Through the analysis of coral from the subtropical waters of Dry Tortugas National Park near the Florida Keys, it has been discovered that there is a 60-75 year-long cycle of ocean warming and cooling. This pattern has been occurring since the 1730s. This cycle is called the Atlantic Multidecadal Oscillation. By measuring the strontium-to-calcium ration in corals, scientists are able to determine past sea surface temperatures. By using a dentist's drill to collect and analyze samples, they were able to see sections showing as short as one month ago, and going as far back as 1837. With the help of two other corals' records, the scientists were able to track a total of 278 years' worth of surface temperatures, going all the way back to 1733.