According to modern building terminology, the shell is any laminar membrane capable of enclosing space and supporting loads through the exertion of direct stresses resolved in the boundaries of the structure. Therefore, domes and vaults, acting in direct compression, are shells. The history of the origin of shells begins about five thousand years ago at Saqqara in Egypt. Little or nothing remains now of any shells constructed by the ancient Egyptians of the Old Kingdom, but the builders of the Middle Kingdom, some thousand years later, roofed their cave tombs with flat compression arched vaults and a few hundred years later, Minoans and Cretans built their beehive domes. Romans did not invent the round or segmental arch but they brought the art of arch making to a peak of perfection making aqueducts, coliseums and the like. During the Middle Ages, cathedral builders began construction of the pointed and groined vaults of the Gothic Period. The beginning of the Renaissance brought with it a revival of dome construction with Michelangelo's dome of Saint Peter being an excellent example. The Renaissance gave way to the Age of Reason with its emphasis on creative thinking and finally to the materialism of the Industrial Age. In our own age, Felix Candela has done much to encourage the use of shell structure. His first shell structure grew out of a design problem solved by the hyperbolic paraboloid shell.