“Periodically we can get voids forming under the roads”

A short iol report sums it up – “Belfast street fall[s] into large hole”. The BBC report includes this explanation

Roads Service spokesman Colin Brown said the city is built upon a deposit of soft clay, silt and mud known as “Belfast Sleech”. “Periodically we can get voids forming under the roads,” he said. “Sometimes it’s very clear what has caused it, other times it’s quite a mystery. “It could be a substantial hole in the road but we’ll know more later.”

Although you’d have to suspect there might be a connection with this

The depression in the road is above a storm-water tunnel in the multi-million pound Belfast Sewers Project.

Work has been ongoing since 2005 on constructing a six-mile network of tunnels running underneath the city.

The key junction in the sewerage network is located at Cromac Street, in the area of the collapsed road.

Updated here.