Task: Question Generation
Input (English): Over time, Roman architecture was modified as their urban requirements changed, and the civil engineering and building construction technology became developed and refined. The Roman concrete has remained a riddle, and even after more than 2,000 years some Roman structures still stand magnificently. The architectural style of the capital city was emulated by other urban centers under Roman control and influence. Roman cities were well planned, efficiently managed and neatly maintained.[citation needed]
Output (English): What Roman construction material has continued to be a mystery to this day?