A trope sometimes seen in movies and TV series (especially in the 80's, somewhat less nowadays) is that aliens come from another star system here in order to harvest resources that they don't have. Often this is water, sometimes something else. There are lots of problems with that premise (one of them being that it assumes that the Earth is somehow special, that it's one of the extremely rare planets in the galaxy that contains those precious resources in abundance. This couldn't be farther from the truth. For example, and unlike most people seem to think, water is one of the most common compound elements in the universe, and can be easily found in things like comets, moons and planets. The same goes for most other elements. An alien race sufficiently advanced to travel interstellar distances could certainly harvest those resources from almost anywhere.) However, the one thing that most people never think of is the way in which the aliens are supposed to lift eg. mill...