In terms of three-dimensional shapes, the sphere, ellipsoid (an egg shape) and dome are able to withstand the most outside pressure. The arch (contained within these three) is stronger than a flat surface.
The force needed to break an arch shape is much greater than that of a flat surface. Domes, spheres and ellipsoids are able to distribute pressure and weight evenly all over the shape so that no one point takes more weight than any other. If equal pressure is applied around the entire shape, it takes great force to break. This is how birds are able to sit on their eggs without cracking them and how arch-shaped bridges are able to carry a number of vehicles without buckling. Flat bridges require vertical supports to accomplish the same task.