Every year, three million Muslims arrive in Mecca, Saudi Arabia, to complete the hajj, an important series of rituals which must be undertaken by all physically and financially capable Muslims at least once during their lifetime. But three million people crammed into any place is a magnet for disaster, and over the years Mecca has seen multiple tragedies which left thousands of innocent pilgrims dead - plane crashes, fires, crushing incidents. The hajj season of 2015 was forced to deal with two separate tragedies - a horrific crane collapse at the Grand Mosque which left behind a nightmarish scene, and a crushing incident just before the Jamarat Bridge which ended with nearly 2500 people lying dead on the streets of Mina.