The first mosque was a modest thing, and the Prophet helped build it. An open-air rectangle of mud-brick walls, its roof of palm trunks and beaten clay rose barely over head height, and palm-leaf thatch gave the only shade. It took about seven months to finish. This was far more than a place to pray. The mosque served as the community's town hall, its courthouse, and its school, with a raised pulpit, the minbar, from which Muhammad taught and delivered the Friday sermon. In those first years the worshippers faced north toward Jerusalem, until the direction of prayer, the qibla, was turned toward the Kaaba in Mecca. A platform along one wall, the Suffah, sheltered the poorest of his followers, who had nowhere else to live.

Every dynasty that ruled the Islamic world left its mark here, expanding and adorning the mosque in turn. The caliph Umar enlarged it; Uthman rebuilt it in stone. The great transformation came under the Umayyad caliph al-Walid I around 706, who brought in mosaicists and craftsmen and, controversially, demolished the living quarters of the Prophet's wives to expand the building, drawing the graves of Muhammad and the first two caliphs into the structure itself. His renovation introduced the first concave prayer niche, the mihrab, in Islamic architecture, and possibly the first minarets. Abbasids, Mamluks, and Ottomans each added their own walls, gates, and towers, so that the mosque became a living record of Islamic art across the centuries, every ruler inscribing his name into its stone.

The single most recognizable feature of the mosque was a comparatively late addition. The dome over the Prophet's tomb was built in 1818 during the reign of the Ottoman sultan Mahmud II, and in 1837 it was painted the green that gave it its name, the Kubbat al-Khadra. Beneath it lies the chamber that was once the room of Aisha, the Prophet's wife, holding the graves of Muhammad and his two companions and fathers-in-law, Abu Bakr and Umar. A fourth space is left empty, reserved by Islamic belief for Jesus, who Muslims hold will return. Adjacent stands the Rawdah, the strip of ground between the Prophet's pulpit and his tomb that a famous saying calls a garden of Paradise, the most sought-after few square meters in the whole sanctuary.

The mosque of today would astonish its founders. Successive Saudi kings have expanded it on a colossal scale, and a project completed under King Fahd in the 1990s, with escalators and dozens of courtyards, was followed by still grander plans, until the structure is built to hold well over a million worshippers at once. Engineering serves devotion in striking ways: the oldest Ottoman prayer hall is roofed with great sliding domes that glide apart on metal tracks, and the surrounding plazas bristle with giant retractable umbrellas, designed by the German Muslim architect Mahmoud Bodo Rasch, that bloom open to shade the faithful from the desert sun. Almost always open, day and night, the mosque has closed to visitors only once in modern times, as Ramadan arrived during the pandemic of 2020. The rest of the time, a tide of pilgrims flows endlessly around the Green Dome.