|The woo is out there|
|Aliens did it and ran away|
The element with atomic number 115 (temporary name:
Elerium-115 ununpentium) is a hypothetical heavy radioactive element that doesn't exist in nature, although it has been artificially synthesized with about 50 atoms produced or suspected to have been produced since around 2003. Currently, the evidence for a discovery does not yet meet the criteria for "discovery" set by the International Union of Pure and Applied Chemistry (IUPAC). This means that element 115 still retains its temporary systematic name, in contrast to flerovium and livermorium, which surround it as 114 and 116, respectively.
Because element 115 is a very heavy and unstable element, it's very unlikely to ever exist in sufficient quantities and with sufficient stability to participate in chemical reactions. However, it's possible to predict its properties based on the other trends shown in the periodic table. It is part of Group 15 of the periodic table, primarily known for containing nitrogen and phosphorus. However, it's most likely to have properties resembling bismuth, as relativistic effects alter the relative energies of electrons in very heavy elements. Hence, while nitrogen, phosphorus and arsenic exhibit a range of oxidation states up to V (5 lost electrons), it's likely that Uup will only go up to oxidation state III. While oxidation state I is rare in the elements between nitrogen and bismuth, Uup is predicted to have a relatively stable I oxidation state. Upon ionization to this +1 charge, the radius of Uup would be expected to shrink around five times more than with the analogous ionization of bismuth and it is expected to have properties closer to thallium(I) than the known but rare bismuth(I).
 In ufology
All of this would excite only physics and chemistry geeks if not for Bob Lazar, who introduced it to UFO lore. According to him, Element 115 is used in UFO engines to generate anti-gravity. Various UFO nuts and wannabe scientists have taken the idea and run with it. This would provide an interesting way of verifying the UFO stories told by Lazar. Should element 115 be synthesised and shown to be capable of powering anti-gravity engines, Lazar would be proved correct. Obviously, given that Lazar runs a website dealing in chemicals and sales of elements, he was smart enough to pick a number higher than any element discovered at the height of his fame in order to hide it from any scrutiny; no use saying carbon or phosphorus has magical powers, as we have more than enough of that to test it.
Lazar's claims state that bismuth has "unusual gravitational properties" (this is a lie, though it may be a misinterpretation of the relativistic effects that control the chemical properties of heavier elements) and known characteristics of Element 115 are expected to be similar (not that this matters, as the longest reported half-life of the element is 200 milliseconds). The claims further state that the element was pressed into discs, then stacked and fused into a cylinder, then milled down to form a cone, and finally sliced to form the key piece of anti-gravity fuel. Again, this is physically impossible given that the element doesn't exist in nature and has been confirmed to be as highly unstable as all the other artificially generated elements in that region of the periodic table. A few proponents of the claim still rave that there may be a magic "island of stability" (a particular combination of protons and neutrons) that would render this element stable, but no signs of such a region of the periodic table have yet been seen. Some of the elements heavier than uranium possess relatively stable isotopes (on the order of thousands of years) but by the time you get to 100, fermium, even the most stable isotopes last on the order of days and it only goes rapidly down from there. Still, the island of stability is a theoretical entity that is good, real, physics; but even this wouldn't help the claims made about element 115, as expected half-lives in this island are on the order of minutes and seconds, which is indeed relatively stable in a region of the periodic table where the atoms last for milliseconds or less.
If element 115 was synthesised, specifically its predicted stable isotope, more conclusively and shown to have an incredibly short half-life and radioactively unstable (which is pretty much conclusive right now) it would show that powering any device through the use of this element would be impossible, and certainly the 500 pounds that he claimed the US government had in their possession would also be an impossible claim. Literally. As that would consist of around 4.72 × 1023 atoms, and with only 50 atoms ever made from all the collision experiments made on this subject in a decade, this would take some time for the government to procure — many times the age of the universe, or so.