The flying saucer is the shape most associated with the alien presence, and is a term known all over the world. Sometimes the shape appears bell like, with the dome humped up in the center. Humans who recall being ferried about in these space ships report a cramped interior, especially if the ship is normally used by aliens of a smaller build than the average human. The rim of the space ship also seems to be unused space. What is the purpose of the saucer shape, or is this simply an aesthetic choice?
During travel throughout the Universe, there are issues more important than appearances or esthetics. Space ships emerge from a higher density, which allows them rapid travel at a speed exponentially higher than that which a lower density would allow. Coming out of a higher density, the space ship and its inhabitants must expect to encounter all manner of possibilities. Even when setting the coordinates closely, the element of the unknown is always present. The primary gravity center may be anywhere in relationship to the ship - above, below, or even to the side. Therefore, space flight or even local hops incorporate production of an internal gravity center so that the passengers are not tossed about. The saucer shape, or its variant, the bell shape, support the technology for production of a gravity center. It is no accident that the planets in your solar system line up in essentially the same plane. The saucer shape simulates this, with the edges of the saucer acting as the outer planets in a solar system's planetary plane.