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The prolonged breathhold has gradually - just gradually - taken a toll on the young ninja sisters. The pair has found themselves in wider space, most likely a cistern, but despite her careful study, Rokumon's groping hands did not uncover anything than a ragged wooden surface. At this point, any common man would have drowned a long time ago, and as Ren felt the wavering movements of her younger sister's thin limbs, she sensed Rokumon now starting to lose her cool - something unheard of for her even in missions of the most extreme duress. 

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