Summary: If we are to explain UFOs in terms of physics we understand to some degree, yet still conform to witness observations, it seems essential to assume UFOs are capable of generating artificial gravity fields (in GR terms, to manipulate the curvature of the fabric of space-time), much as we produce magnetism with electric currents. An approximation -probably oversimplified- of overall UFO behaviour is via accepting such a (hypothetical, or at least not known to us) gravity-like repulsive "force-field". The glow / luminescence in various colours around the UFO (apparently shape depends on UFO's shape as well as its current maneuver, so the UFO's outline as seen by external observer can change), is thought to be due to ionization of surrounding air (atmosphere around the UFO "lights up", much like what happens in neon-lamps), hereafter referred to as "UFO plasma sheath". Brightness/color changes of the "UFO plasma sheath" seem to be related with thrust/acceleration. The air ionization seems to be caused by the EM radiation emitted by the UFO, and is thought to be a secondary effect of the propulsion system. This includes UV (suggested by many cases of sunburn-like effects and eye and skin irritation) and soft X-rays (suggested by "burn ring" traces on ground where UFOs landed). Considering the difficulty of creating plasma in normal atmospheric conditions, in combination with other observations, like the luminosity of submarine UFOs, the sudden appearance of condensation / mist when starting up in high humidity conditions and noise patterns, it all suggests an envelope of lower atmospheric density near the UFO's surface. A near vacuum, i.e. the air/water is "pushed away" from the skin of the craft (confirmed by observations when UFOs rise from water), which minimizes friction and heating issues. Plasmas can interact strongly with electromagnetic radiation, "Plasma Stealth" is a proposed process that uses ionized gas (plasma) to reduce the radar cross section (RCS) of an aircraft. This may explain why sometimes UFOs are seen visually, but not tracked on radar. Often UFOs have a very strong magnetic field. Also, in several cases, light (e.g. from car headlights or beaming spotlights) is reported to "bend" in front of the UFO, an effect which some suggest is related with the most controversial aspect of UFO reports: the apparent ability to disappear / "blink out" or seem to "implode" (diminish in angular size) [miniature-scale "gravitational lensing"-type phenomenon?]. Physiological effects on humans include: sunburn-type effect and eye irritation, extreme dryness of the nasal area and of the throat, color changes in vision, extreme headaches and a heating/burning sensation. Witnesses and animals have become sick and even died, with symptoms similar to radiation poisoning, after close approach to a UFO.