Abstract: A prefabricated modular building includes a plurality of lightweight contoured panels which are interconnected. Each of the panels has arcuate top, bottom and side edges so that the panels provide identically sized and contoured segments of an oblate spheroid. A securement mechanism is associated with each of the side edges of the panels and the securement mechanism of adjacent panels are interengaged for securing the panels together. The top edges cooperate and define a circular opening of substantial diameter. A fastening mechanism is associated with the top edge of each of the panels and a compression ring is cooperatively engaged with the fastening system of each of the panels for positively accurately positioning and aligning the panels and for thereby providing a rigid structure.