But yesterday visitors to Belfast joined hordes of locals to come and photograph the flood which sensationally showed up the flaws in Northern Ireland's most state-of-the-art road scheme.

One Polish man who has lived in Belfast for three years said that he had never seen a road so badly flooded in his home country.

Standing overlooking the vast expanse of brown water, he said incredulously: "I never saw anything like this in Poland...it's just unbelievable."

Open-topped tour buses slowed as they drove past the underpass, bemused tourists leaning over the top and pointing their cameras into the watery abyss.

Regional Development Minister Conor Murphy arrived yesterday afternoon at the spot where weeks before he had pronounced the road open.

Environment Minister Sammy Wilson joined him and the pair walked down to the water's edge to inspect the very obvious problem.

Although a study in diversity – Mr Wilson in a charcoal jumper and Mr Murphy in a black pinstripe suit – the two politicians agreed that the rain had been unusually heavy for an unusually long period.

But both men appeared as stunned by the catastrophe as the tourists taking pictures above them.

They praised the men beavering away in the background to empty the vast swimming pool and spoke to the firefighters monitoring the giant pumps.

But Mr Wilson said that his well-known scepticism about increased carbon dioxide being responsible for damaging climate change was unshaken by the deluge.

"Those people (who believe we are responsible for climate change) would say from their models that this should be happening less often because this part of the northern hemisphere should be getting dryer," he insisted.

After the ministerial delegation had departed, the visitors kept coming, some parking their cars on nearby roundabouts

In the sunshine, children played in the spray from a holed hose pumping water out of the flooded cavern, oblivious to the headache for road planners.

But on Monday, as commuters into Belfast grind slowly along the makeshift route around the underpass the tough questions for those who engineered this gleaming Westlink centrepiece are likely to intensify.