Located under the hill called the Citadel of Kaymakli, this underground cave-city is the widest one in the whole region. It was opened to visitors in 1964.
More info http://www.goreme.com/kaymakli-underground-city.php
Address (Unnamed Road), Kaymaklı 50760, Turkey
Coordinates 38°27'35.735" N 34°45'8.958" E
