The Great Mars Tragedy was a tragic event in the course of Mars' terraforming which had cost 4,876 people's lives.
In 2125 Mars already had an atmosphere suitable for human use, but two days before declaring the terraforming complete, tragedy struck. A "chemical miscalculation in the planetary atmospheric processors made the new atmosphere unstable" and led to it being unable to "stick" to the surface of Mars.  Unfortunately, the people of Mars had already stopped using breathing apparatuses, the loss of atmosphere happened "so fast that no one was able to sound an alarm or seal the ventilations of the various installations around the planet.". 
List of victims