BEIJING—Duan Mengmeng peered into a dark tunnel flooded with water and spotted a potential exit. The problem: A series of metal pipes blocked the way.

Ms. Duan, 25 years old, and her five friends—all young professionals in their early 20s—fanned out looking for other ways to escape. Liang Ying, a 23-year-old office administrator, knocked on the walls. Zhang Xiaobao, 24, who works in the media industry, used his smartphone to illuminate the floor and ceiling.

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