The vast majority of South Africa's gold lies deep underground but atop the mine dumps scattered around Johannesburg, you can actually walk on the stuff.

You're standing on gold right now, said Charles Symons, chief operating officer for DRDGold , as he surveys a yawning crater 30 meters (90 feet) deep and 250 meters long blasted out of the Elsburg dump to the south of Johannesburg.

But the specks on your shoe wouldn't buy you a piece of bubble-gum, he says with a laugh as a water cannon in the distance blasts more dirt from the side of the crater.

For decades, South African miners have been extracting residual gold from the dumps, comprised of crushed rock and other waste material pulled out of mines in tunneling and blasting operations.

Some of the mines date back to the original gold rush that gave birth to Johannesburg more than a century ago and would have been dragged there by mules. Since extraction methods were also primitive, a lot of gold also got tossed into the rubbish.

Sophisticated chemical methods have enabled companies to re-mine the dumps, Johannesburg landmarks that resemble giant hills of sand. But even 21st century technology only enables them to get around a third of the residual gold.

Gold's record run means there are fat profits to be had if you can squeeze more gold out, and companies like DRDGold are on a scientific quest to do just that.

GOLDEN NEEDLES IN GIANT HAYSTACKS

Don't think about hiking up a Johannesburg mine dump, shovel in hand, expecting to strike a mother lode.

For every metric tonne of solid waste heaped in the dumps there is just 0.3 grams of gold, or 0.3 parts per million.

But with gold fetching over $1,700 an ounce, that can add up. At 140 million tonnes, the Elsburg mine dump contains about 1.3 million ounces of gold, worth about $2.2 billion at current prices.

DRDGold currently only extracts about 37 percent or 0.12 grams per tonne but Symons says the company is closing in on a method to raise that level to 55 percent, or 0.16 grams per tonne. That means an additional 2,300 ounces of gold a month.

We can activate it in a laboratory so we are hopeful it can work in the plant, he told Reuters during a visit to the plant.

Work also still needs to be done to determine if the technology can be implemented in a way that is cost effective.

The process starts with the giant water cannon blasting debris from the dump at a pressure that would kill a person.

Water mixes with the waste material in the dumps, also known as slimes. This mixture is piped from the dump and goes through an elaborate chemical process that ends, improbably, in a gold bar.

In the initial stage, screens remove or sift organic material from the sludge. It then gets piped into vast tanks in which the gold is dissolved by coming into contact with cyanide in the presence of oxygen and lime.

In the next phase it is adsorbed by carbon, a chemical process by which it accumulates on the surface.

The solution then goes through an electro-winning process where a current is passed through it, precipitating the gold onto an electrode. From there it is removed and smelted and then presto -- you have a gold bar.

FREE THE GOLD!

The key to harvesting more gold, Symons says, is to bring as much gold as possible into contact with the cyanide.

The lab work has therefore focused on isolating sulphides in the slimes, since sulphides have much higher concentrations of gold, and find-grinding the material to expose as much gold as possible to the cyanide.

If we could grind it down fine enough we could liberate all the gold. But that is just not possible now, said Symons.

Still, any extra bit will just go to the bottom line.

It's a no-brainer to extract gold from dumps in this price environment. Any increase in recovery will just add to the bottom line provided the new technology is cost-effective, said David Davis, a mining analyst at SBG Securities in Johannesburg.

A rise in DRDGold's output is hardly going to shake markets. South Africa's fourth largest gold producer, its output is only around 265,000 ounces a year and Ergo contributes about 50,000 ounces of that.

But its efforts are part of a wider technical race to extract as much gold as possible while the price is red hot.

With South African mines the deepest in the world, much research is being channeled into how to safely reach and haul out the ore from the bowels of the planet.

AngloGold Ashanti , the world's third-largest gold miner, aims to use new technology to go 5 km (3 miles) below the surface to unlock 70 million ounces.

Gold Fields is looking at robots and other technology to improve safety. Its chief executive strongly hinted in a recent interview with Reuters that the company would soon make a major technological announcement.

GREEN SPIN-OFF

Squeezing gold from the mine dumps is also seen to have a beneficial impact on the environment as the massive mounds that litter the Johannesburg outskirts get flattened.

The Ergo unit, formerly an Anglo operation, leveled 64 mine dumps from 1977 to 2005, extracting 8 million ounces of gold.

Some would argue the benefit is that widely-scattered dumps are removed from populated areas exposed to their toxicity and dust. But other experts say the benefits are limited.

I would say the environmental impact is neutral. It doesn't make the problem any worse but it doesn't improve it, said Dr. Terence McCarthy, a professor at the School of Geosciences at the University of the Witwatersrand in Johannesburg.

They are just making a few mega-dumps, he said.

At DRDGold's Ergo plant, for example, once the slimes has been treated it is pumped several miles to what is called the Brakpan Tailings Dam -- essentially a mega-dump that also holds the water gets recycled for the whole process.

McCarthy said rain that drained through the dumps still added to a problem of highly acidic or toxic water in the region.

The mega-dumps were destined to be re-mined as technology advances even farther, McCarthy said.

I can tell you those mega-dumps will be re-mined again because the next thing they will take out of there is the uranium. And if they can improve the techniques to get more of the gold out they will do that too, he said.

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