FIVE days after firefighters put out the underground fire that closed roads in central Johannesburg, cut power to vast areas in the business district and cost the city millions in damaged infrastructure, the tunnels beneath the city still looked like a blackened hell.

The fire started at about 7pm last Sunday and was only extinguished 18 hours later, leaving the subterranean tunnels that carry many of the city's electrical backbone submerged in blackened pools of foetid water.

The electrical substation on the corner of Bree and Loveday streets is where most most firemen, medics and repair technicians enter this world. Technicians blink as they emerge to the surface from a manhole in the middle of the road, their overalls black as coal from the carbon dust emitted by the burning cables.

They wear high-tech masks to keep the dust out of their lungs. Experts have already cleared the poisonous gases that kept repairmen out of the tunnels even after the blaze was killed.

About 60 technicians, general workers and electricians have been working around the clock since Wednesday morning to restore power to the city centre. But the narrow tunnels keep all but 12 on the surface until their turn comes to replace exhausted colleagues down below.

The teams have already removed more than 15 tons of burnt cables since the tunnels were declared safe two days after pumping out most of the water used to put out the fire.

By yesterday the teams had cleared the damaged cables from about 200m of tunnel.

The acrid smell of burnt cables still assails the nose. A pale fluorescent light guides the visitor to the entrance, but technicians have strung up strings of bulbs to light the blackened underworld below the city's streets.

Louis Pieterse, head of engineering operations for City Power, guides us through the turns and crannies of this secret world. Our path is dry, but moisture still seeps from blackened, broken cables that hang from every surface.

Pieterse soon stops us, preventing us from going to the uncleared areas where his men are still struggling to clean up the mess.

"Sorry, sir, it turns out that there is double the damaged cables that we are yet to see," said a technician bringing bad tidings to the boss.

A power generator makes communication difficult.

"The cause of the fire is still unknown but cable theft has always been our biggest problem," Pieterse says.

He knows about the homeless people who make their way into the substation to escape the cold.

"We are introducing electronically controlled covers to close all the tunnels," he says.

One piece of good news: Pieterse expects all the city's residents will have their power restored by 11pm.