Flash-flooding in Belfast over the weekend saw the recently completed Westlink underpass submerged in 6m of floodwater.
The flooding has prompted the launch of an investigation by the Northern Ireland Roads Service.
Regional Development Minister Conor Murphy ordered the Roads Service to launch a full investigation following an emergency meeting at Stormont.
The new underpass is expected to remain closed for most of today as emergency services try to pump out the floodwater. It is thought the flooding was caused when a pumping station beside the road was overwhelmed when a nearby river overflowed.
West Belfast councillor Tim Attwood said "serious questions" needed to be asked about why the newly constructed underpass was submerged in 6mof water.
"The Roads Service has serious questions to answer about how the new Westlink underpass could be flooded so quickly," he said.
Northern Ireland Water's customer relations centre dealt with more than 4,000 calls over the weekend after one month's rainfall lashed the province in just 12 hours on Saturday.