The latest point of contention is an artificial reef. Spain is accusing Gibraltar of using it to keep Spanish fishing vessels away from waters belonging to Gibraltar. But Gibraltar says the reef serves only to protect marine animals. It's an argument Spain knows all too well, because environmental protection also plays a role in the dispute over ships refueling on Gibraltar. Because fuel is tax-free on the 6.5-square-kilometer outcrop, several million tons of it are stored and sold on Gibraltar. Spain says that's too great an environmental risk.