We use scanning force microscopy and stepwise erosion in a radio-frequency (rf) plasma to obtain a three-dimensional volume image of an ultrathin poly(styrene-b-butadiene-b-styrene) triblock copolymer film. The free surface of the film exhibits domains of two distinctly different structures, i.e., hexagonal arrays of isolated dots and parallel stripes. A common forehand interpretation relates these surface morphologies to a cylindrical microdomain morphology with the cylinders oriented perpendicularly and parallel to the plane of the film, respectively. In contrast, our experiments show that the "dots" merge into parallel cylinders at some distance underneath the film surface. These results demonstrate that care has to be taken when the thin film morphology of block copolymers is inferred from their surface structure.