One of Mashhad’s best kept secrets is the Mahdi Gholi Beyk bathhouse, also known as the king’s bath, which is one of the biggest Persian bathhouses. The building consists of different sections; the most interesting one is sarbonyeh or changing room. Sarbonyeh has a large dome resting on eight columns made out of stone, and is decorated by paintings in many superimposed layers. The oldest layer dates back to the Safavid dynasty (16th-18th century) and the newest one belongs to the Qajar dynasty (18th-20th century). The bathhouse contains unique structural features such as: skylights for taking advantage of the natural light; copper containers on burning wood for heating water and outlets for the smoke produced.