Elerium-115

Elerium-115 is a fictitious element featured in the X-COM video game series, inspired by conspiracy theories surrounding Ununpentium. It is said to contain high amounts of energy. In the first X-COM game it is only acquirable by capturing intact alien Power Sources (50 units of E-115/Power Source) and cannot be manufactured, thus making it one of the most valuable items in the game. In later games, it can be extracted in mines on other planets.

Enemy Unknown
In the first installment of the series, Elerium is first discovered when scavenging intercepted UFOs. It is used by aliens for propulsion and for their special 'plasma' weapons. Elerium is said to give energy by the following way: if it is bombarded with accelerated particles, part of it will transform to antimatter; the matter-antimatter mutual annihilation is the reaction which gives the energy, but the reaction also emits gravity waves and a functioning Elerium-powered reactor also generates its own gravity field, independent from all outside gravitational interference.

Later in the game the player also develops his own elerium-powered weapons called Fusion Ball Launcher (following the example of the alien weapon Blaster Launcher) and Plasma Cannon (based on reverse-engineered plasma technology, specifically researching Plasma Rifle+clip or Heavy Plasma+clip enables researching Plasma Beam). It is depicted in the game as an orange crystalline substance, but incidentally it has the same graphic as that of a Stun Bomb in the Battlescape (purple ball).

Terror from the Deep
In the second installment of the series, another element called zrbite is used by aliens in a similar way as elerium but seems to be a bit richer in supply. According to fanon, it works by amplifying sonic waves while giving off large amounts of energy. It is part gold, part alien biometal; because of the latter, it cannot be manufactured, since some of its components are unknown to human science. When the Second Alien War ended, all Zrbite on the world has become inert, since the aliens' molecular control network was the key to unlocking it's power. However, it isn't forgotten; in 2054, the Transtellar Corporation unveils the prototype for the Mark I Conversion Drive, an ion propulsion system that generates power through helium-carbon fusion. The key to the success of the design was the use of a reaction containment chamber with a shell composed primarily of inert Zrbite.

Apocalypse
In X-COM: Apocalypse, the third installment of the series, Elerium is used to power all engines and most weapons and can be bought from a corporation known as Solmine, which mines it from Mars. Due to it being available quite affordably through the game, acquiring it is never an issue.

Interceptor
In X-COM: Interceptor, the fourth installment in the series but taking place third in the timeline, a synthetic form of Elerium-115 is developed and is considered weapons grade, allowing the deployment of various new weapons systems. During the game prior to this, the use of the element for military purposes other than power generation or propulsion was prohibited due to the limited supplies.

Although research from the first installment says that Elerium is not naturally found inside the Solar System, in October, 2061, after several weeks of transmissions, the Mars probes show an indication of what could be a huge stockpile of Elerium-115 several thousand kilometers from Cydonia. Plans are immediately set in motion to send a manned expedition to Mars-the first since the final X-COM cleanup mission in November, 2009. The Elerium mined there was more than enough to satisfy Earth's needs and as such, Transtellar has purchased all rights to interstellar research abandoned when Elerium stockpiles from the first war ran out. In a matter of months, Transtellar finished plans for a prototype hyperdrive, tested successfully in 2065. This advancement paved the way for X-COM: Interceptor.