It's one of the pillars of Islam that all Muslims who are financially and physically able must perform this journey at least once in their lifetime.
I've personally been to the Hajj three times -- in 2005, 2006 and 2007. I went not as a pilgrim, but as a producer covering the event for CNN -- one of the most challenging but rewarding assignments I've ever been given.
It's a breathtaking sight to watch the sea of people at Mecca's Grand Mosque circling the Ka'aba, the black cube-shaped building. It's believed the Ka'aba stands on the spot where Abraham built his first temple to God and, while the building itself is not sacred, it is a spiritual symbol. It is towards this direction that Muslims around the world orient themselves to pray five times a day.
Right across the Muslim world, hotel rooms will have a sticker somewhere with an arrow pointing towards the Ka'aba, so the visiting faithful know which way to pray. It's the proverbial North in a Muslim's compass.
So what makes the Hajj so unique?
It's truly global
Sitting there on the white marble floor of the Grand Mosque, it was difficult not be blown away by the diversity of the people passing by. Groups of Indonesians in crisp white wearing colored headbands for identification and moving in tight phalanx formations quietly chanting the mantra of the Hajj (which translates approximately to "Oh God, I have obeyed your call").
Groups of West Africans in colorful garb, almost singing verses from the Quran. Old Chinese couples, groups of blonde Europeans and Americans; it felt as if we were watching the entire world walk past.
The Devil's makeover
One of the key rituals of the Hajj is called the "stoning of the devil." Part of the cathartic process of the pilgrimage is to throw stones at three pillars known as the "Jamarat," which symbolizes the rejection of the devil's temptation. This was also the most dangerous part of the ritual when it came to crowd control -- imagine 3 million people throwing stones at the same time in the same location.
Many people have been wounded -- even killed -- by stones lobbed by pilgrims further back. The Saudi authorities spent millions of dollars renovating the area, making it multi-layered like a garage instead of one flat plain, and renovating the pillars themselves that represent Satan.
When I first visited Mecca, the "Devil" was an obelisk-like pillar, but the renovations included replacing the structure (after much religious scholarly debate) with a wide oval walled structure with a much bigger surface area that is easier to hit.
Saudi Arabia is a religiously conservative country that practices a very strict interpretation of Islam, which includes a rule that unrelated men and women should not mingle in private spaces.
So our female reporter and producer technically can't be in the same room (or tent, once we're out in the desert) as the male cameraman.
The various compromises with authorities included leaving the hotel room door wedged open at all times, leaving the tent flap open, sometimes having a token chaperone in the room in the form of a government minder, or just occasionally being dropped in on to make sure we were actually working and not misbehaving.
However, I did appreciate that women and men all pray together and perform all the rites together at the Hajj -- whereas mosques are segregated. Men and women are only segregated for their sleeping arrangements.
Women in Saudi Arabia -- including female visitors -- have to wear a long-flowing black robe (an abaya) and a headscarf that covers their hair. In many malls, hotels and restaurants in big cities like Jeddah and Riyadh, women can get away with removing their headscarves. But in Mecca, during Hajj, these rules are strictly followed.
One late night as we were frantically trying to make an edit deadline, I received a call from an interviewee I had agreed to meet in the hotel lobby. I rushed down and walked out of the elevator and within seconds realized that every single person in that lobby was staring at me in horror. I had forgotten to throw on my abaya and headscarf and was wearing only jeans, T-shirt and a ponytail -- which is comparable to walking around the Vatican in a bikini.
But there have also been times when the Hajj has turned into a nightmare.
The day the apocalypse arrived
It was the last day of the Hajj in 2005.
We were in our hotel room overlooking the Grand Mosque as the pilgrims performed the final rites as they circled the Kaaba. The sky began to darken and the windows shook with the force of roaring thunder, and the torrential rains started. We went out among the crowds and the scene was almost movie-like. Many exhausted pilgrims caught up in the moment of their spiritual journey claimed "Judgment Day" had arrived and that we were witnessing the apocalypse.
The grounds of the mosque were flooded, the tent city at Mina was hit with landslides, cars and buses were turned on their side -- it was utter chaos. It turned out not to be the apocalypse, but a sobering reminder of what can happen when a desert city without a drainage infrastructure suffers torrential rains while 3 million people happen to be in town.
In 2006, as the crew was headed to the airport thinking our assignment was over, we received word that a stampede had taken place. In the rush to try to beat the crowds on the last day chaos ensued and some 350 people were trampled to death. We came back to the sounds of ambulance sirens wailing in warning and family members wailing in mourning.
Just a few hours earlier, there was a palpable sense of collective euphoria as pilgrims completed their rites and were spiritually "cleansed" and ready to go home. Now the scene was of carnage: blood, bodies shrouded in the same white cloths that they had performed their pilgrimage in. At the time, it was the deadliest day at Hajj in years.
Subsequent pilgrimages avoided a similar disaster by spreading out the times that people can conduct the stoning ritual, and carefully controlling the number of people at the Jamarat at any one time to avoid bottlenecks.
But in 2015, more than 700 people lost their lives
and hundreds more were injured in another stampede. Among the suggested causes: pilgrims rushing to complete the rituals, heat, masses of faithful pushing against each other in opposite directions, even confusion among the many first-timers on the annual Islamic pilgrimage to Mecca and Mina.
But despite past tragedies, despite the crowds, despite the traffic, the experience of standing on the plain of Arafat makes is one that will always remain with me. The Day of Arafat is the spiritual culmination of the Hajj, the peak of spiritual cleansing, as millions of people shed tears as they pray for God's forgiveness for their sins.
There are few sights to equal the sea of people in white praying and crying in the most effusive expression of religious emotion.