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.