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Africa's wealthiest city was built on gold. Fortunes were made, mansions were built, thousands of miners toiled and a metropolis was created.

But today, after 120 years of mining, the gold is disappearing, hundreds of mine shafts have been abandoned, and Johannesburg is facing a nightmare of biblical proportions: a vast tide of poisonous water rising inexorably toward the foundations of the city itself.

The problem is known as "acid mine drainage" and it has affected mining regions around the world, including Canada. With nearly 6,000 abandoned mines across the country, South Africa is more endangered than any other. But no metropolis is as threatened as Johannesburg, and the situation here offers a glimpse of what might some day happen in parts of Canada if the problem is not properly managed.

Some analysts are calling it a ticking time-bomb. One scientist has described it as "South Africa's own Chernobyl" - potentially one of the worst environmental catastrophes in the continent. The toxic mine water is 550 metres below Johannesburg, but it is rising by at least 0.35 metres a day - possibly up to a metre a day - and could reach a critical level within the next 15 months. Officials have acknowledged that the rising water could begin to trigger sinkholes and earth tremors if it reaches within 150 metres of the surface.

The implications, according to environmentalists, are like something out of a disaster movie. The acidic water could corrode the foundations of Johannesburg's buildings, eating away at the steel and causing buildings to collapse. Small earthquakes could be triggered as water flows into cracks in the earth. Drinking water could be contaminated. Rivers and wildlife reserves could be endangered. Huge sinkholes could be created. Drainage from old uranium mines could cause radioactivity in the water supply. Human health could be damaged.

More than a century after the first gold boom, South Africa's miners are digging deeper and deeper to find the remaining bits of gold in their waning industry. Some of their mine shafts now extend nearly four kilometres below the earth's surface - the deepest in the world. As the minerals disappear, thousands of mines have been abandoned by their owners, leaving the government to struggle with the drainage threat.

Acid mine drainage is a phenomenon that begins when rising groundwater floods into old mine shafts and tunnels. During mining operations, this water is normally pumped out. But when the mines are abandoned, there is nothing to stop the water from flooding through the mine shafts. This water causes an oxidation of metal sulfides in pyrite in the surrounding rock, and the resulting product is a highly acidic water - often filled with toxic heavy metals - that decants toward the surface as the mine shafts become deluged.

Johannesburg is particularly vulnerable because there are millions of cubic metres of water in the natural aquifers below the city. The clean water is now becoming contaminated by the toxic mine water as it rises.

Already the toxic water has bubbled to the surface in rural areas near Johannesburg, polluting rivers and ponds. In the Krugersdorp wildlife reserve, two hippos are believed to be slowly going blind because of acidic and toxic water in their pond, contaminated from an old mine shaft.

Mine water could also threaten the famed Cradle of Humankind, the ancient caves near Johannesburg that contain fossil evidence of one of the birthplaces of the human species.

As a short-term measure to protect Johannesburg, the city is planning to spend more than $30-million on pumping stations so that any flood of mine water can be pumped away. But critics say the government has been too indecisive and slow to act. "Up until now, there's only been crisis management," said Mariette Liefferink, a South African environmental researcher who has become an outspoken campaigner on acid mine drainage.

"The government is merely lulling the public into complacency and apathy," she said in an interview. "That failure to make a decision could lead to disaster."

Lance Greyling, an opposition MP in the South African Parliament, has warned that millions of litres of acid mine water are already flowing into wetlands and rural areas near Johannesburg. He said it is an "environmental crisis" that the government is neglecting.

In Canada, unlike South Africa, there has been co-operation between governments and the mining industry to prevent a catastrophe from acid mine drainage, Ms. Liefferink said. She attended a conference on acid mine drainage in Nova Scotia last week and was impressed by the Canadian response. But if anyone in Canada is tempted to slacken their efforts against the acid mine water, they only need to look at South Africa to see the potentially disastrous consequences, she said.