Residents of south and east Belfast and commuters from further afield were last night bracing themselves for traffic chaos after a giant hole was found under one of the city's main roads.
A statement from NI Water said a "significant void" was discovered yesterday and it was too early to tell when the road will reopen.
It is believed the problems centre on an old Victorian sewer at Ravenhill Road.
"During the course of a major sewer rehabilitation project in the Park Road/Ravenhill Road area, contractors working on behalf of NI Water have discovered a significant void," a spokeswoman said.
"NI Water is liaising with Roads Service colleagues in relation to the matter. A temporary closure of the road will be required to allow work to address the situation to be completed."
The public will be able to access updates via the internet.
"Further details will be carried on the Trafficwatchni website and updates will also be carried on the niwater.com," the spokeswoman added.
"Due to the unforseen nature of the problem the extent of the work required to repair the road fully is not yet clear and as such the longevity of this closure has not been established."
It was unclear last night if the work could be completed as quickly as today or if it could take up to two weeks, as some residents fear.
Today, cars travelling into Belfast city centre are expected to be diverted onto the Ormeau Road, at the Rosetta roundabout, while country-bound traffic will be diverted via the Ormeau Embankment.
Joanna Bateson lives close to the hole and is concerned about the disruption on her doorstep.
"These works have been going on since the middle of June and are set to last for three months, but this latest development will cause more chaos," she said.
Another resident, who did not wish to be named, was angry at the ongoing disruption.
"We have to put up with this until September but it is very hard. Park Road for a start has been a virtual building site."
Motorists were dreading disruption. One regular commuter, on his way home to the Newtownbreda area of south Belfast, feared being clogged up in traffic.
Another said: "The Ravenhill Road is vital for people getting in and out of the city and it is vital it is reopened as soon as possible."