A concrete shell, also commonly called thin shell concrete structure, is a structure composed of a relatively thin shell of concrete, usually with no interior columns or exterior buttresses. The shells are most commonly monolithic domes, but may also take the form of hyperbolic paraboloids, ellipsoids, cylindrical sections, or some combination thereof. The first concrete shell dates back to the 2nd century. Uses Most concrete shell structures are roofs. Concrete shell construction techniques are well suited for complex curves and are also used to build boat hulls (called ferroconcrete). Historically, it was used by the British to create the Mulberry Harbours for the 1944 D-Day invasion of Normandy. Advantages Like the arch, the curved shapes often used for concrete shells are naturally strong structures, allowing wide areas to be spanned without the use of internal supports, giving an open, unobstructed interior. The use of concrete as a building material reduces both materials cost and construction costs, as concrete is relatively inexpensive and easily cast into compound curves. Steel is often used in conjunction with concrete to reinforce the structure. The resulting structure may be immensely strong and safe; modern monolithic dome houses and the Miami Marine Stadium withstood hurricanes, and concrete shell structures survived the 1985 Mexico City earthquake. Concrete shells are widely considered to be strong enough to withstand even F5 tornadoes. Disadvantages Since concrete is a porous material, concrete domes often have issues with sealing. If not treated, rainwater can seep through the roof and leak into the interior of the building. On the other hand, the seamless construction of concrete domes prevents air from escaping, and can lead to buildup of condensation on the inside of the shell. Shingling or sealants are common solutions to the problem of exterior moisture, and dehumidifiers or ventilation can address condensation. Historic concrete shells The oldest known concrete shell, the Pantheon in Rome, was completed about AD 125, and is still standing. It has a massive concrete dome 43m in diameter, with an oculus at its centre. A monolithic structure, it appears to have been sculpted in place by applying thin layers on top of each other in decreasing diameter. Massively thick at the bottom and thinning (with aerated volcanic pumice as part of the concrete mix) at the top, the Pantheon is a remarkable feat of engineering. Modern concrete shell construction Modern thin concrete shells, which began to appear in