Why was A&E at Belfast's Royal Victoria Hospital on its knees?
For several hours on Wednesday night, Belfast's Royal Victoria Hospital was "at breaking point", some members of staff said.
It declared a major incident because the back log of patients was so high.
But what is worrying was that this was no major trauma. There had been no major car accident or fire; there was no flu epidemic or winter vomiting bug.
All of this happened against a relatively quiet backdrop. So why?
There is no real explanation for the huge surge in patients and the "chaotic" scenes that followed.
The Royal Victoria Hospital is Northern Ireland's regional trauma centre housing its biggest accident and emergency unit.
It has an international reputation for its pioneering work dealing with the victims of 30 years of Northern Ireland's Troubles.
However, it is understood that problems had been simmering since last weekend when patients had been diverted from other hospitals to the RVH.
By Wednesday, the RVH was still not back on its feet and it had to start diverting ambulances to other hospitals.
But how did Wednesday's crisis happen?
The lack of middle-grade doctors across the United Kingdom is partly to blame. There is a 50% shortage affecting emergency departments nationwide. The lack of medical staff is a real problem that is being felt across Northern Ireland.
It has also been suggested that the majority of medical students are women. Five to seven years down their career paths, they may marry and have children. Given the hours and the stress, some of them may not choose a career in the emergency department and may decide to leave. Men, too, want a work/life balance and it is difficult to recruit them to a career in an emergency department.
Staff burn-out in A&E is another issue.
In November 2012, almost all of the emergency medicine consultants in the Belfast Health Trust raised concerns about the safety of A&E. The clinical director of the RVH at that time, Dr Russell McLaughlin, also decided to step aside. It is understood he felt strongly about the running of the A&E.
Another problem for emergency departments is the four-hour target for patient waiting times. There is tremendous pressure on staff and on individual health trusts to tick that four-hour box.
What must also be addressed is the provision of minor injuries units and out-of-hours services that the community feels confident about. What role are GPs playing to support that and to prevent Accident and Emergency being used for anything other than genuine accidents and real emergencies?
Questions about what happened at the RVH on Wednesday night must now be put to the trust, senior managers, the health and social care board and its chief executive John Compton.
But ultimately one question must be put to Health Minister Edwin Poots.
Is the current policy on accident and emergency working in Northern Ireland?