System, method, and apparatus for harnessing geothermal power from superhot geothermal fluid (SHGF) and magma reservoirs. An exemplary embodiment is directed to a cased wellbore includes a well casing suspended within a borehole that extends between a surface and an underground reservoir of magma and a boiler casing housed within the well casing and extending between the surface and the underground reservoir of magma. The boiler casing has a first end submerged within the underground reservoir of magma and a terminal end opposite to the first end. The cased wellbore also includes a fluid conduit housed within the boiler casing and configured to deliver a liquid-phase fluid to the terminal end of the boiler casing. A temperature and a pressure at the terminal end of the boiler casing converts the liquid-phase fluid into a gas-phase fluid that travels through the boiler casing towards the surface. The cased wellbore also includes a well head connected to the first end of the boiler casing.