Abstract: For the exploitation of the geothermal energy in an active submarine volcano an elongated, substantially vertical, columnar concrete body is arranged above the orifice of the volcano so as to extend from the water surface vertically downwards through the water and into the magma in the orifice of the volcano so that the lower portion of the concrete body is submerged in the magma. The concrete body has such a displacement and such a weight that it floats in a balanced vertical position in the water and the magma. A coolant, preferably water, is circulated through internal cooling ducts or pipes in the concrete body from the upper end of the body downwards into the lower portion of the concrete body, which is submerged in the magma and where the coolant is heated by heat transfer from the surrounding magma, whereafter the heated coolant is returned through internal cooling ducts or pipes in the concrete body to the upper end thereof, where the heat content in the heated coolant is utilized.