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Rows of colorful expedition tents line a sandy bank beside a turquoise underground river inside Hang En cave, Vietnam's third-largest cave. Dramatic shafts of sunlight stream in through the enormous cave entrance in the background, illuminating the cathedral-like chamber and the rocky terrain beyond the pool. The immense scale of the cavern makes the tents look miniature, and swifts, the birds the cave is named after, can be seen darting through the sunlit entrance.