On a beach walk last year in northern Mayo, Ireland, I thought I had found a spectacular fossil seaweed. Close inspection showed that instead it was a kind of photo image of a sargassum-type weed on a Dalradian metavolcanic rock. There were other examples nearby, all at the mean high water level and all of the same species of weed on the same rock type. I took the rock home and, a year on, the image is still crisp (see photo). However, all attempts to imitate the conditions have failed. Do readers have any idea of how the image occurred, and are there any other instances of natural photoetching?Tony Legg, St Martins, Jersey