Media playback is unsupported on your device Media caption David Shukman takes a look inside the world's largest iron ore mine

In the heart of the Amazon rainforest in Brazil, giant diggers tear into the rock 24 hours a day to extract a dark grey ore rich in the iron on which every modern economy depends.

The Carajas complex is the largest iron ore mine on the planet and at any one time 3,000 people are toiling here in the tropical heat using a fleet of giant machines including trucks the size of houses.

Amid an ocean of lush green jungle, a series of four manmade chasms stretching deep into the rock represents an ugly first step in the long process involved in making steel.

The company operating the mine, the Brazilian giant Vale, often criticised for causing environmental devastation, claims it is planning to restore this landscape to its original state.

The conundrum of Carajas is that we all make use of steel but that comes at a price to the natural world.

Constant struggle

This is the source of a constant struggle between Vale's desire to reach new seams of ore and attempts by the environmental authorities to keep the expansion under control.

It is also at the core of the debate on sustainable development at the Rio+20 summit under way this week.

I reported on a similar contest in the UK last week, over plans for the expansion of Lydd Airport in Kent close to the wildlife reserve of Romney Marsh.

Image caption The caves are thought to contain unusual biology

The iron rush here at Carajas was only triggered by a chance discovery.

An American geologist, whose helicopter needed refuelling at Carajas in 1967, reputedly bent down to retie a shoelace and noticed the Amazonian soil littered with chunks of rich ore.

The lumps he found here, almost as black as coal, are surprisingly heavy - I picked one up - because this ore has one of the highest iron contents anywhere in the world.

Huge operation

Nearly half a century later, the mine processes a staggering 300,000 tonnes every day and last year generated an immense total of 109 million tonnes - snapped up by the fast-industrialising economies of Asia.

What was once a landscape of dense vegetation is a now a moonscape of bare cliffs and billowing dust.

At first sight the mining operation appears breathtakingly destructive.

For a start the mine is smack in the middle of a National Forest and what was once a landscape of dense vegetation is a now a moonscape of bare cliffs and billowing dust.

The whoops and cries of jungle birdsong are replaced by the constant roar and grind of hundreds of massive engines.

But Vale, like most multinationals these days, is eager to promote the idea that sustainability is embedded in its thinking and points to a series of measures designed to limit the mine's impact.

Media playback is unsupported on your device Media caption David Shukman goes inside the lost caves of the Amazon

The operations manager of the mine, Jaymilson Magalhaes, tells me that the mine complex only covers about 3% of the area of the national forest and that before any digging can start, the company has to have a restoration plan to return the area to its original state.

That includes using spoil to fill in the mines once they are exhausted to reshape the topography - a process we witnessed in one small area - and undertake a massive replanting programme using native species.

"I believe we genuinely can restore the forest and we have a strategy to do that," Jaymilson tells me.

"What we do is very careful planning so that when we finish we know exactly the plants we need to replant and we have nurseries with the original vegetation.

"When we grow them we will reposition the soil so the forest can grow back to its original state."

Trying to be green

Image caption The area is one of great natural beauty

Vale also highlights its support for an extensive monitoring operation in the forest run by the Brazilian Government's conservation agency ICMBio - so we checked with the agency to get their perspective.

Frederico Drumond Martins of ICMBio is the manager of the national forest and agrees that Vale is trying to be greener - for example, he says, he only has 12 rangers but Vale pays for a further 80, plus cars, boats and the use of a helicopter, all vital to guard against illegal logging and poaching.

"Vale is really trying to operate sustainably but there's a long way to go - for Vale the iron comes first and Nature second or third."

Frederico and his colleagues are locked in a series of disputes with Vale over its plans for new mines in the forest.

"My job," he tells me, "is not to stop the mining - it is good for the economy and it puts Brazil in a good position in the world - but it is to control it."

Precious caves

One of his greatest concerns is to preserve a surprising and recently discovered world beneath the Amazon - a series of caves lurking in the iron ore under the forest floor.

In this one region, some 2,000 caverns have been found and scientists regard them as potentially precious features because of their iron content, unusual biology and archaeological remains.

A cave we descended into hosts four species of bat - only one of them carnivorous, luckily - and excavations in its floor have revealed evidence of human habitation as long as 9,000 years ago.

The air inside was cool and musty and there was a constant squeaking from the bats as they fluttered above our heads.

The iron is for this generation but the forest is for the next generation Frederico Drumond Martins, ICMBio

ICMBio and Vale are surveying the caves to rank their importance - only those granted the highest grade will be saved from mining while some may be destroyed if others are preserved.

The status of the cave we visited has yet to be decided so its fate is unclear.

It lies within a zone identified for potential mining by Vale but any bid to start digging will require a lengthy planning battle.

The rainforest is under assault from a variety of sources and, compared to soya planting and cattle grazing, iron ore mining causes relatively minor damage.

And there's an irony: the vehicles used by the conservation rangers around are made with steel that may have had its origins in this very landscape.

Expansion of the mines would create new jobs and lead to valuable exports. An informal estimate of one planned project is that the ore could yield, at current prices, a staggering $800bn.

Set against that is a growing awareness of the uniqueness of the forest, not only with the ecosystems thriving within and below its canopy, but also a dark and largely unknown realm under the forest floor.

As Frederico of ICMBio puts it: "The iron is for this generation but the forest is for the next generation."

As the host of the Rio+20 summit this week, Brazil faces its own difficult choices over how to define the much-disputed phrase 'sustainable development' and what it means for the jungles and caves of the Amazon.