The Portugal shoreline makes for some amazing scenery and geological structures. One of the most prominent scenes that dots this landscape is the deep earth that opens up as craters along the shore. A sign nearby explains that these are formed by the waves that erode the earth as they crash upon the high cliffs. The land is so high that the land under the surface will erode away and create these water caves under the earth closest to the water. When the cracks on the surface open up, you get these amazing craters that form. Be sure to not get too close as this land is not stable.