Aliens: Extinct Races (Codex)
 Primary Codex Entries
Fifty thousand years ago, the Protheans were the only spacefaring species in the galaxy. They vanished in a swift "galactic extinction". Only the legacy of their empire remains. They are believed to have built the mass relays and the Citadel, which have allowed numerous species to explore and expand throughout the galaxy.
Prothean ruins are found on worlds across the galaxy. While surprisingly intact for their age, functioning examples of Prothean paleotechnology are rare. Time and generations of looters have picked their dead cities and derelict stations clean.
Some believe the Protheans meddled in the evolution of younger races. The hanar homeworld of Kahje, for example, shows clear evidence of Prothean occupation. The presence of a former Prothean observation post on Mars has caused a rebirth of "interventionary evolutionists" among humans. These individuals believe the god-myths of ancient civilizations are misremembered encounters with aliens.
 Rachni (Mass Effect)
The second species to join the Citadel, the salarians are warm-blooded amphibians with a hyperactive metabolism. Salarians think fast, and move fast. To salarians, other species seem sluggish and dull-witted. Unfortunately, their metabolism speed leaves them with a relatively short life span; salarians over the age of 40 are a rarity.
The salarians were responsible for advancing the development of the primitive krogan species to use as soldiers during the Rachni Wars. They were also behind the creation of the genophage bioweapon the turians used to quell the Krogan Rebellions several centuries later.
Salarians are known for their observational capacity and non-linear thinking. This manifests as an aptitude for research and espionage. They are constantly experimenting and inventing, and it is generally accepted that they always know more than they're letting on.
 Secondary Codex Entries
 Protheans: Beacon
The beacon was a Prothean artifact unearthed on the Alliance colony of Eden Prime. Its resemblance to the Prothean data banks recovered on Mars provoked immediate interest from the Alliance and the Citadel Council. It proved to be a solid state data storage device, part of a galaxy spanning comm network similar to the modern extranet. Intact Prothean "paleo-technology" is rare, the beacon seemed to promise another quantum leap of technology, akin to the discovery of the mass effect drive and relays.
Unfortunately, the beacon also drew the attention of the rogue Spectre Saren Arterius and his synthetic allies, the geth. A dawn raid by his flagship Sovereign resulted in hundreds of civilian casualties. The beacon was badly damaged. The motives behind the attack are still being investigated.
During the recovery operation, the beacon fired a pulse of energy at the Executive Officer of the Alliance frigate Normandy. Lieutenant Commander Shepard survived and appears to have suffered no ill effects. Afterwards, the beacon fell inert. The mechanism appears to be dead.
 Protheans: Cipher
The Prothean beacon downloaded its knowledge into Lt. Commander Shepard on Eden Prime, causing confusing dreams and visions. While the imagery is becoming clearer with time, the meaning of the beacon communication remains elusive.
It has been suggested the Prothean data recording is highly dependent on a certain point of view, what Carl Jung described as the collective unconscious. The "cipher" needed to comprehend the images implanted in Shepard's mind is the cultural knowledge of a Prothean: the archetypes, biological instincts, and common experiences universal to the race. Since the Protheans have been dead for millennia it may be impossible to acquire this "cipher".
 Protheans: Data Discs
Despite all the evidence confirming the existence of the Protheans, little is known about their culture and society. From time to time, dig sites will yield new clues, but after 50,000 years of decay, little of value is unearthed.
Recent research has focused on the discovery of Prothean data discs. On their own, they are frail and rarely found in one piece. Occasionally, however, an intact disc will be discovered within a console or reading device.
To date, over three dozen discs have been recovered and a few of those have been restored to the point where researchers can begin analyzing them. Though it may be some time before scientists discover a way to transfer the data off the discs, they are currently considered the most tangible leads for learning more about the Prothean culture.
 Protheans: Martian Ruins
After twenty years of manned research outposts and nearly a century of robotic exploration, the European Space Agency's Lowell City became the first permanent settlement on Mars in 2103. Within a decade, the United States and China had founded permanent settlements, as well.
The south polar region of Promethei Planum developed a "Bermuda Triangle" reputation. Satellites detected intermittent mass concentration and magnetic field shifts. In 2148, prospectors working near Deseado Crater discovered an underground complex: a Prothean observation post. The odd phenomena were generated by the operation and discharge of a mass effect core, struggling to function despite fifty millennia of neglect.
Earth was electrified with the news. Humanity was unequivocally not alone. While courts battled who owned the ruins, the international scientific community coordinated a massive effort to access, translate, and interpret the databanks recovered from the facility.
The facility proved to be a biosciences observation post built when Homo sapiens were first evolving on Earth. While the motives of the Protheans are not certain, translated records indicate that the facility was in regular communication with automated observation platforms in Earth orbit and the lunar nearside. The half-dozen mass effect spaceships found in the facility were presumably used for first-hand observation.