Baleen whales are characterized by having baleen plates for filter feeding and two blowholes. Baleen whales have two blowholes, as opposed to toothed whales which have one. Marine erosion on rocky coastlines produce blowholes that are found throughout the world. Humpbacks have a , heart-shaped bushy blow through the blowholes. Dolphins are voluntary breathers, who must deliberately surface and open their blowholes to get air. A tourist attraction, the fountain depicts whales spouting water out of their blowholes. Their nostrils, or blowholes, are situated at the top of the cranium. The blowholes and dorsal fin are often exposed above the water surface almost simultaneously. There are also stacks, natural arches and blowholes. Often the blowholes and dorsal fin are visible at the same time. It has twin blowholes with a low splashguard to the front. Tiny gas bubbles are called porosities, but larger gas bubbles are called "blowholes" or "blisters". They also aided their owners in finding seals by alerting them to seal blowholes. Falling into one of the blowholes would be almost certainly fatal. This is evident, for example, in the blowholes of numerous marine mammals.