The Hajj is the annual Muslim pilgrimage to the holy city of Mecca. Muslims who are physically and financially capable of performing the Hajj are expected to do so at least one in their life. When I lived in Senegal, I knew people who had saved their money for decades in order to make the journey. They were highly respected.
In 2015, hundreds - probably thousands - of pilgrims lost their lives in the Hajj, crushed or suffocated to death in what has been termed a "stampede" but was probably a crush precipitated by two very large groups meeting at an intersection due to the blockage of one route of travel. There has not been an official explanation of the factors that led to the tragedy.
The New York Times assembled an excellent scrolling account that follows one survivor and attempts to understand what led to such a catastrophic loss of life among people performing one of the most exalted acts of their religion. [How One of the Deadliest Hajj Accidents Unfolded]