Element Zero, also known as "eezo", is a rare material that, when subjected to an electrical current, releases dark energy which can be manipulated into a mass effect field, raising or lowering the mass of all objects within that field. A positive current increases mass, a negative current decreases it. This "mass effect" is used in countless ways, from generating artificial gravity to manufacturing high-strength construction materials. It is most prominently used to enable faster-than-light space travel without causing time dilation. When humans discovered the Prothean ruins on Mars, they also discovered refined element zero that the Protheans had left behind. It enabled research into FTL ship drives before the Charon Relay was discovered.
Eezo is generated when solid matter, such as a planet, is affected by the energy of a star going supernova. The material is common in the asteroid debris that orbits neutron stars and pulsars. These are dangerous places to mine, requiring extensive use of robotics, telepresence, and shielding to survive the intense radiation from the dead star. Only a few major corporations can afford the set-up costs required to work these primary sources. Some planets have small eezo deposits or coalesced around a larger deposit during their formation. While these secondary sources are safer to mine, the yield from the ore is not as large. There are rumours that the Nemean Abyss has particularly rich eezo deposits.
Dust-form element zero is often released after engine accidents. It is used by many species to influence or strengthen the presence of biotics. If a child is exposed to dust-form element zero in utero, due to its mutagenic effects, there is a small chance they can develop eezo nodules throughout their nervous system that react to electrical stimuli from the brain. This allows them to use biotic abilities, but many exposures have no effect, or result in terminal cancer. There is some question about just how "accidental" some of those initial exposures were, after the link between eezo and biotics was established.
Element zero forms the basis of many advanced medicines throughout Citadel space. The biotic drug red sand allegedly has element zero as its base; according to urban legend, it was created by criminal triads on Mars from the eezo samples recovered there. The SSV Normandy's massive Tantalus drive core cost 120 billion credits because of the amount of element zero needed to power the stealth system.