A group based decision making exercise on what is killing coral reefs. It considers the impacts of humans on the natural environment and other influences. Students have to work in groups to decide what is killing coral reefs. There are statements that are to be used as clues and some red herrings. There is a question sheet and final response sheet to the murder mystery. There are suggestions of specific group roles. There is a lesson plan, demonstrating differentiating and assessment of learning, Designed for KS3 but could be used as an introduction lesson to coral reefs at GCSE.