An older, car-sized dwarven tunnel network crossing the whole floor, used for moving massive dwarven diggers quickly.
When a Dispel strips away Carl's smoke, it also cancels the roof fortifications overhead, and Carl, Donut, Rosetta, Simoom, and Gonk all drop a dozen feet into a huge passage. It is clearly not something built by Justice Light's team but an older tunnel built and fortified by the Semeru Dwarves — wide enough for cars. Carl remembers Juice Box telling him these passages intersect the entire floor so the massive dwarven diggers could get from point A to B quickly.
Two dozen dwarves of Team Retribution are scattered through it, showing as white dots with green ally outlines. One heals Gonk; two stand over the corpses of the vanished Samurai Rhino Guard. A dwarf shouts that the dispel was strong enough to cancel the roof fortifications, another re-forms the roof and plunges the tunnel into darkness, and a third reports enemies fell in further down the line — including a war mage. A massive explosion later knocks everyone off their feet and starts a partial collapse, until blue light and grunting dwarven magic keep the bulging ceiling from buckling.
A cavernous earthen passage as wide as a road tunnel, roofed by dwarven fortification magic that can be re-formed on command, plunging pitch dark once sealed.
It serves as the escape route and fortified fallback line that saves Carl's group from the enemy army.