Blowhole in Fortaleza de Sagres, Sagres Point, Sagres, Algarve, Portugal, Europe

On the point of the Sagres headland you will find a good example of a blowhole, a natural, vertical vent which rises from the roof of a cavern in the base of the cliff right up to the cliff top. When the sea is rough you hear the waves pounding into the cavern, whooshing air upwards and then sucking it back down again. In geology, a blowhole is formed as sea caves grow landwards and upwards into vertical shafts and expose themselves towards the surface, which can result in blasts of water from the top of the blowhole if the geometry of the cave and blowhole and state of the weather are appropriate. A blowhole is also the name of a rare geologic feature in which air is blown through a small hole at the surface due to pressure differences between a closed underground system and the surface. The blowholes of Wupatki National Monument are an example of such a phenomenon. It is estimated that the closed underground passages have a volume of at least seven billion cubic feet. Wind speeds can approach 30 miles per hour. Another well known example of the blowhole is the natural entrance to the Wind Cave.