The Ruins of Mars:

After the Thirty Day War on Earth (which led, within mere years) to the formation of the Terran Federation, space exploration increased drastically, spurred by increasing population pressures, the removal of self-destructive cruft in government, and the dominance of space power in ending the Thirty Day war in the first place.  The secret Lunar Base provided the first new fuel of the 'Second Space Race', which was He3, 'mined' from lunar rock on the sunny side of the moon and used in primative fission rockets.  

Within 30 years the first Terran Federation manned expedition to Mars had begun. Originally conceived as a propaganda victory, to showcase the bold new future of mankind, the mission was both a daring success and a miserable failure.

Shortly after the first teams landed they discovered signs of life, or former life, buried under the red sands and rocks.  Faced with proof that intelligent life had once lived on Mars, the Terran Federation quickly shut down the propaganda arm of the mission and began restricting colony efforts to military personnel... and to offset the loss they opened the Moon to civilian operations, to civilian 'Helium Miners' to fuel this new space industry.

The uncovered martian ruins were kept a secret for centuries, until the very collapse of the Terran Federation at the end of the "Atomic Era', and are still barely known to the common citizens of the Imperium.  

The Ruins were extensive, for the native Martians had settled their entire world, building vast canals, cities and more.  They hadn't entered space, not meaningfully, and resembled humans quite closely based on images and a tiny handful of well preserved corpses. Their genetics, however, were purely alien... parallel evolution at work.  These Martians had died more than 50,000 years earlier from a total civilization collapse which was aggrivated by an environmental collapse.  However, aside from a few new material technologies, there was very little to be learned from the Martians, whose own technology never quite equalled the task of entering space, and in fact appeared to be stuck in pre-atomic physical science, though their grasp of engineering and their extensive use of silicon-plastic compounds was actually in advance of terran technology at the time.

WIth the initial reason to keep the discovery secret gone (the hope for advanced technology... the TF was still young and unstable at the time), the Federation decided not to reveal the Martian Ruins for new reasons.

First, of course, was the potential spiritual crisis when Man learned that they were not the first advanced race in our solar system. The second was the existential terror that the sudden, total failure of the Martian Ecosystem represented for Earth.  Stellar Geologists were able to determine that at the height of Martian Civilization the temperature and atmosphere were a lot closer to Earth norms, with no explainable reason for the loss.  It seemed, once the martian language was deciphered, that their own scientists had been equally baffled.

However, the lessons learned about xeno-linguistics and more well prepared the Federation for first contact, even if the Martians couldn't answer any deep secrets of the universe.



The question that is asked, of course, is how the secret was kept for so long? Mars became a major stopping point for Asteroid Miners, at the Olympus Mons 'spaceport', after all.  A good portion of the answer lies in the location of the spaceport, of course, and the fact that the most obvious ruins were well buried by martian sand long before men arrived. It was relativly simple to use subeterranian explorations, to rebury the biggest cities, and smaller digs drew no attention whatsoever. Most of the experts on Mars lived on Mars itself, spending their entire lives there, with only a handful retiring to earth.  Wildly popular speculative fiction about the martian civilization was released as a sort of innoculation against leaks, and later appeared, shot in real martian ruins, as a series of popular action movies... meaning that visible cities could then be explained away as sets.

And within a generation the first relativistic ships were headed out to explore truly alien worlds, and mars, and whatever secrets it held, was quickly forgotten.



In the Imperial Era, it is believed that the Martian Civilization was wiped out by a more advanced race before they could attain spaceflight, though the exact nature of the weapon is still speculative.  Though natives of Mars are well informed about Martian ruins, in fact inhabititing many of the 'lost' cities, the rest of the Imperium is kept relatively in the dark, barring a few archeologists of high standing.  The most remarkable aspect of the Martians, aside from the manner of their extinction, remains the parallel evolution. Martians would have been unable to survive on Earth, unable to consume the native plants or animals and struggling with the hot, humid and 'heavy' air compared to their homes, and they could never have evolved into Humans of any sort, but aside from truly minor cosmetic differences they looked remarkably human, and based on their records, acted remarkably human as well.

So far, no one has a good answer for why or how such a remarkably human like race could have appeared on a planet so very close to Earth (the acknowledged homeworld of all the branches of humanity in the galaxy), even as humans were just starting to appear in a familiar form on Earth.  The mystery may never be solved.