The Hajj, the fifth pillar of Islam and one of the largest religious pilgrimages in the world, is currently taking place in Mecca, Saudi Arabia. Starting earlier this week, and continuing through Monday, approximately 3.4 million Muslims from around the world are participating in several rituals, including the Tawaf -- the circumambulation of the Kaaba, the enormous cube-shaped building in the center of the Masjid al-Haram mosque. The pilgrims are are also taking part in the Sa'i, traveling back and forth between the mountains of Al-Safa and Al-Marwah, and Ramy al-Jamarat, in which they throw pebbles at three walls in the city of Mina to show their defiance of the Devil. The rituals and locations date back to the 7th Century, but much of the surrounding area has grown and changed radically over the years. For comparison, I've included several historical images of Mecca as well, dating back as far as 1889. The rest of the photos depict this year's Hajj and the pilgrims on their sacred journey.