In the photo above you see the caldera of Golovnin volcano located in Sakhalin, Russia. In fact it was an ordinary volcano that had been sleeping and erupting until its central part vanished under the ground as if it never existed. A big pit
surrounded by high walls appeared in that place instead. A new volcano (small one) is growing in its center. It actually looks like a lake but there is a volcanic activity inside that may result in a new eruption quite unexpectedly.