Mercury's Terminator city destroyed by impactors
Terminator, Mercury's main city, rides on rails around the planet and stays in the habitable band just ahead of the lethal sunrise. The heat of the sun expands the tracks, and that expansion pushes the city forward. When a coordinated swarm of small, hard-to-detect impactors destroys it, Swan Er Hong and Wahram survive by walking for days through a utility tunnel beneath the surface to stay ahead of the dawn. The attack shows that a settlement engineered around one precise physical niche is catastrophically fragile to a single deliberate strike.
