Every year, on the 12th and last month of the Muslim Lunar calendar, what is one of the world's largest displays of unity among humans takes place in Saudi Arabia as more than 2 million Muslims perform their obligation of a pilgrimage to the city of Mecca. Known as the haj, the pilgrimage is the fifth pillar of Islam, and any Muslim who is able to (in financial and health terms) is expected to attend the haj at least once in their lifetime. Last year saw 724,000 Saudis attend the haj, with a further 1.65 million pilgrims coming into Saudi Arabia from more than 187 countries. The pilgrimage brings with it a swathe of issues for the host kingdom. Providing transport for pilgrims is a challenge. The 7km journey between holy sites can take up to 10 hours by bus, and the Executive Committee for Pilgrims' Transportation licenses nearly 14,000 extra buses to operate during the haj season. The situation looks set to improve though, with the announcement that a monorail system is to be built, linking the holy sites in the Mecca region. At a cost of US$1.9 billion, the monorail will consist of four to eight carriages and will be able to transport about 20,000 pilgrims an hour. A total of 800,000 pilgrims will be able to use the service over the course of the haj. The project is expected to be completed within the next four years. Almost 200,000 pilgrims were treated at emergency medical facilities during the last pilgrimage. The Saudi Ministry of Health mobilises 9,000 doctors and other medical personnel each year to cope with the demand. More than 100 health centres are set up at the holy sites of Mecca, Mina, Arafat and Muzdalifah.