I designed this UFO for a building competition I entered with some other people. We built an alien invasion themed built and we made it to the top 10 out of 170 teams. We needed several alien ships and a few military vehicles to protect our town. Instructions on how to build some of those vehicles, like a tank and fighter jet, can also be found on this site.
I personally love this UFO due to the combination of iron blocks and obsidian, though unfortunately it often looks horrible in other texture packs, so you may want to use different blocks.
The full instructions are pretty much all covered in the image below, you can click on it for a large version. The UFO isn’t difficult at all. You only have to create 5 layers, which start out small and increase in size twice. These new layers are never placed on top of blocks of the other layers.
I also added some “computer screens with an alien language” by placing signs inside the UFO, though it won’t always be visible, depending on how you built it.
The UFO’s are built in such a way that water or lava can be placed in the middle of the glowstone blocks, this can make it look as if the UFO’s are either firing a laser beam or using a tractor beam to pull up mobs, because they float upwards while they’re in the water beam.
I sometimes also use these UFO’s on my crop fields (sometimes with a crop circle), as they can look quite funny and their water beams can be used to hydrate the farmland.