1. More accurately, it is a flooded fumarole, an opening, crack or hole, in the Earth's crust, generally located within the vicinity of a volcano, which emits steam and gases escaping from molten lava below.

2. In a typical year some form of unrest (earthquakes, ground deformation, change in fumarole activity, or eruptions) occurs at about 18 large Calderas worldwide, and eruptions occur within or near at least five of them.