 As a coral grows it'll grow a layer every year so similar to this section at the top and it incorporates what it's feeling around it so the temperature of the water the rainfall that's happening around it and we can work out what the temperature has been in the past from that and how the rain has affected the landscape as well from that. As it grows it incorporates geochemical indicators so oxygen and strontium and calcium to its skeleton and we can drill little holes through the coral and we can work out exactly what it was like one week ago or 400 years ago from the coral.