JOHANNESBURG - The City of Johannesburg has confirmed the CBD explosion was caused by gas.
But they are yet to determine the source of that gas.
The city has also ruled out illegal mining.
Human rights lawyer Richard Spoor believes sewer gas is the cause of the explosion.
He says this is likely due to poor ventilation in the tunnels and has significant consequences.
"These kinds of explosions only take place when you get an explosive mixture of gas in the tunnels," he said.
"You get these kinds of mixtures if the tunnel is not properly ventilated so the risks of explosions is well-known and well understood."