I am not a fan of re-posting work that has been done by others, however I recently watched a video by Dark5, a popular Youtube channel, that covered Project Aurora in depth. In their traditional style of video you must read the details as the video plays, however it is definitely worth the six minutes of your life if you are interested in alternate propulsion that may be in use by our governments.
Magnetohydrodynamics (MHD) is quite the mouthful, but seems to be one of the latest forms of propulsion and may help explain a huge variety of UFO sightings. MHD relies on magnetic properties of electrically conducting fluids to provide lift and thrust. Some common fluids used are plasma, liquid metal, salt water, and electrolytes.