Abstract: An underground transformer cabin consisting of two prefabricated vibrated reinforced concrete units, one inside the other. A larger external unit is enclosed by a covering plate, and a smaller, internal unit is raised off a bottom of the larger external unit so that there is a wide air space between the sides and bottom of the two walls. The bottom of said smaller internal unit has windows or openings through which fresh outdoor air flows into the smaller internal unit from underneath, passing first over the transformer and then into an outlet pipe which extend externally at a height above the inlet pipe.