Nancy Drew: City Under The Basement (Book 18)

Nancy and her father Carson find themselves in an ancient city long buried below a luxurious estate, the sale of which has brought them to Turkey. Thieves led by a mysterious rich man named Harold Severino search frantically for a priceless artifact somewhere in the city as Nancy and her father are taken prisoner. Severino, who previously attempted to purchase the estate with suspicious motives, has been led to the archeological treasure by none other than Professor David Sever, the crooked archeologist from Nancy Drew #2: Writ in Stone! It's up to Nancy to halt the plundering of this living museum and somehow make it out in one piece!