Blowhole formed by the remains of a dissolved fossil palm tree, near Neapoli, Greece. The process starts when organic material becomes buried in sediment, but doesn't get dried or preserved. Instead this material is eaten away and replaced by minerals that crystalize along the tiny pores that form in the organic material. Eventually these minerals are dissolved away by rainwater, which is mildly acidic, leaving a hole where the tree once stood. The hole shown here is roughly 30cm in diameter, close to the sea shore and connected to the sea via channels deep under the surface. In rough weather seawater is forced up the hole at great pressure producing a column of spray some meters above the surface.