The moment in AD 2465 when the first permanant interface between the earthshield and the overground was established by the Amtrak Federation above Houston/GC. This marked the beginning of the Federation's crusade to reclaim the surface world from the Mutes.
Trackers had been making prolonged recon missions on the surface since not long after the Federation's founding, but it was only after the Break-Out this process really began accelerating, with the prototype wagon-trains and aircraft appearing a few decades later.