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When it comes to hidden underground cities, few people were better builders than the Turks: The Derinkuyu complex in Turkey's Cappadocia region could provide refuge for at least 20,000 people in its carved-out caverns — but now archaeologists are surveying a site they think could be even bigger: a cave complex beneath a Byzantine-era hilltop castle in nearby Nevsehir that may go 370 feet (113 meters) deep.