The Grand Mosque has undergone extensive urban development and mega projects that are commensurate with its sanctity and the high number of visitors it receives daily. The Saudi leaders have always shown great care and attention towards the Grand Mosque, starting from King Abdulaziz bin Abdul Rahman, who unified the country, to the era of his sons, and now in the prosperous era of the Custodian of the Two Holy Mosques, King Salman bin Abdulaziz, and the Crown Prince. There is ample evidence of this care for the Two Holy Mosques in all aspects that enrich the visitors" experience and provide them with comfort and facilitation. The engineering aspect, especially in the 3rd Saudi expansion, is one of the most prominent aspects that reflect the state"s concern for the Grand Mosque. The expansion was designed with architectural engineering details derived from the ancient Islamic heritage while keeping pace with modern design principles and harnessing technical capabilities to ensure the purpose is achieved aesthetically and practically. Hamoud bin Saleh Al-Iyadah, Undersecretary of the General Presidency of the Affairs of the Grand Mosque and Prophet"s Mosque, explained that the third Saudi expansion included 22 domes, of which 12 were movable glass domes, six fixed glass domes on the second floor, and four fixed domes on the middle halls on the second floor. The portable dome is located at the top of the honorary corridor of the third Saudi expansion building, with an external diameter of 36m, an internal height of 25m, and a weight of 800 tons. It has internal and external facades of marble and glass for the openings, with the external surfaces made of colored mosaics and the internal ceilings made of wood inlaid with precious stones. The glass domes open through a central control unit to provide natural ventilation when the temperature is appropriate. Al-Iyadah added that the 12 movable glass domes are located above the back and middle courtyards, with dimensions of 28.5 x 17 meters, an external height of 8.75 meters, and a weight of about 300 tons. The fixed glass domes are located on the second floor with a total of six units, with dimensions of 9 x 14 meters, an internal height of 35.5 meters, an external height of 2 meters, and a weight of about 20 tons. Al-Iyadah added that the 12 movable glass domes are located above the back and middle courtyards, with dimensions of 28.5 x 17 meters, an external height of 8.75 meters, and a weight of about 300 tons. The fixed glass domes are located on the second floor with a total of six units, with dimensions of 9 x 14 meters, an internal height of 35.5 meters, an external height of 2 meters, and a weight of about 20 tons. Additionally, four fixed domes are located on top of the existing middle halls on the second floor. Their external diameter is 16.6 meters, internal height 9.6 meters, external height 13.6 meters, and the weight 25 tons per unit. These fixed domes have external and internal facades of marble and glass for the openings, with external surfaces of colored mosaics and internal ceilings made of wood inlaid with precious stones. — SPA