In cities large and small around the global, there's a whole other world lying beneath the streets. Shaped by history, weather and geography, these underground cities are fascinating foundations and reflections of life above. While many of these subterranean worlds have long since become obsolete, others are fully functional urban spaces. If you're intrigued by what lies under your feet, check out Cheapflights.com's picks for the top 10 underground cities around the world.
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