How does Carl get trapped in the world-spanning dungeon?

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What happens

1. The Transformation An alien corporation called the Borant Corporation triggers the “Transformation” — Earth’s buildings collapse and are repurposed into a subterranean, multi-level dungeon. The entire planet becomes the World Dungeon under Borant’s control. All human-constructed structures on the surface are destroyed or swallowed into the Dungeon.

2. Stairwells appear After the Transformation, 150,000 illuminated stairwells randomly open in densely inhabited regions of Earth. These stairwells are the only portals into the Dungeon. If someone enters one, they become a “crawler” — a participant in the Dungeon Crawl game show.

3. Carl enters with Donut Carl, formerly a U.S. Coast Guard marine technician, survives the Transformation by being outdoors when everything collapsed. He goes in search of his ex-girlfriend’s cat, Princess Donut, who escaped their apartment just before everything collapsed. After finding her, Carl and Donut descend a stairwell into the First Floor, officially becoming crawlers.

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So in short: an alien corporation turns Earth into a massive dungeon; portals (stairwells) open; Carl enters one in pursuit of Donut as everything changes; once he enters, he’s trapped in the Dungeon with no way back up.