Original: The Hassan II Mosque, located in Casablanca is the largest mosque in Morocco and the third largest mosque in the world after the Masjid al-Haram (Grand Mosque) of Mecca and the Al-Masjid al-Nabawi (Prophet's Mosque) in Medina.
Simplified: It is the largest mosque in Morocco and the third largest mosque in the world after (i) the Masjid al-Haram (Grand Mosque) of Mecca and (ii) the Al-Masjid al-Nabawi (Prophet's Mosque) in Medina.