Florida professor Subrata Roy has analyzed UFOs and he thinks he know how they work. He wants to build a saucer-shaped craft that can hover like a helicopter, has not moving parts, and uses no fuel– it will propel itself using electrodes that cover its surface and ionize the surrounding air into plasma.
The biggest hurdle will be building a craft large enough to carry passengers that is still light enough to use this energy source.
In Scientific American, Larry Greenmeier quotes Roy as saying, "All of the materials needed to make this aircraft currently exist, and plasma is the most abundant form of matter in the universe. If we can somehow tap into that in the future we should be able to fly anywhere." (to other planets?)