There have been numerous incidents during the Hajj, the Muslim pilgrimage to the city of Mecca, that have caused loss of life. Every follower of Islam is required to visit Mecca during the Hajj at least once in his or her lifetime, if able to do so; according to Islam, the pilgrimage is one of the Five Pillars of Islam. During the month of the Hajj, Mecca must cope with as many as three million pilgrims. Plane travel makes Mecca and the Hajj more accessible to pilgrims from all over the world. As a consequence, the Hajj has become increasingly crowded. City officials are required to control large crowds and provide food, shelter, sanitation, and emergency services for millions. Despite these efforts, incidents have still occurred. Crushes and failures of crowd control Sometimes the dense, surging troupes, trekking from one station of the pilgrimage to the next, cause a stampede, or more precisely, a progressive troupe collapse. At densities above six to seven persons per square meter, individuals cannot move, groups are swept along in waves, individuals jostle to find breath and to avoid falling and being trampled, and hundreds of deaths can occur as a result. The Stoning of the Devil (ramī aj-jamarāt) ceremony is particularly crowded and can be dangerous. Pilgrims ritualistically throw pebbles at three walls (formerly pillars before 2004) which represent the three places where the Hadiths describes how the devil tempted Abraham. It is one of a series of ritual acts that is performed during the Hajj. Some notable incidents include: 2 July 1990: A crush inside a pedestrian tunnel (Al-Ma'aisim tunnel) leading out from Mecca towards Mina and the Plains of Arafat led to the deaths of 1,426 pilgrims, many of them of Malaysian, Indonesian and Pakistani origin. 23 May 1994: A crush killed at least 270 pilgrims at the Stoning of the Devil ritual. 9 April 1998: At least 118 pilgrims were trampled to death and 180 injured in an incident on Jamaraat Bridge. 5 March 2001: 35 pilgrims were trampled to death in a stampede during the Stoning of the Devil ritual. 11 February 2003: The Stoning of the Devil ritual claimed 14 pilgrims' lives. 1 February 2004: 251 pilgrims were killed and another 244 injured in a stampede during the stoning ritual in Mina. 22 January 2005: A stampede through the stoning ritual in Mina led to the killing of three