Ed Conway, Economics Editor

"When you've played the dirt long enough, you can tell how it's gonna fall."

With that, the man sitting by my side turns to the rock face, pulls on a joystick under his thumb and all hell breaks loose.

In front of us, the sheer outcrop dissolves in a juddering series of smacks and thuds.

He pulls the lever again and the enormous bucket of our digger tears more rocks off the cliff-face.


Are Britain's trade figures as golden as they seem?

As he demolishes the outcrop, scoop by scoop, larger and larger landslides jolt us and threaten to engulf the cabin. I start to feel a little nervous.

"Nah, don't worry," says the operator. "We don't get buried all that often."

He points at the mammoth truck in front us - bigger than, well, a house.

"You wanna feel what it's like in that cabin when you have a hundred tons of rock land on you."

Our digger scoops up a massive bucket full of stones, swings it across to where the truck is waiting and, with a deafening crash, drops the rocks onto its back. Yes, I conclude, rather here than there.

Image: One or two grams of gold are extracted for every ton of rock mined

This is mining, 21st century style. We are sat in the cabin - nest might be a better word for it - of a vehicle that stands three storeys high, and the man in the driving seat is, quite literally, tearing this mountain apart.

One could doubtless find more sophisticated ways to describe it, but the more time you spend at a modern gold mine, the more you realise that ultimately what it amounts to is dismantling a mountain and squeezing some precious metal out of the remains. It is a paradox - primitive and technologically-advanced at the same time.

There is Wi-Fi throughout the underground mines, and yet the way they signal an emergency is by releasing a "stench" gas throughout the tunnels - something they've been doing here in mining country for the best part of a century.

Image: The tunnels in the mine are wide enough for trucks and have Wi-Fi

These days there are few gold mines with open seams in them - those mines they show you in the movies.

Most major gold deposits instead contain microscopic traces of gold - maybe one or two grams of the stuff for every ton of rock you tear out of the ground. You never see it in the ground. You just play the numbers game: the more rocks you shift, the more gold you might get.

Here at the Cortez mine in Nevada, operated by Canadian giant Barrick, that means shifting a lot of rocks - comfortably more than the weight of the Empire State Building every single day, if you can get your head round that.

Image: Rocks weighing more than the Empire State Building are mined daily

It means trucks bigger than anything you've seen before, with wheels taller than a double decker bus and engines pumping out 4,000 horsepower.

Image: Some of the mining vehicles' wheels are as big as a double decker bus

It means deep underground mines with tunnels big enough to drive a truck through.

It means detonating hundreds of thousands of pounds of explosives, digging up the rocks, grinding them in enormous mills, pulverising them into dust, mixing them with cyanide solution, adding carbon, heating the resulting sludge up to more than 1,000 degrees and only then do you see it: molten gold.

There is something hypnotic about finally seeing it, after all that work.

After watching what they call "the pour" - when the refiners meld one gold bar after another, I found myself chatting with the underground mine superintendent, Trent, and noticed him looking a little wistful.

He confessed he had never even seen any of the gold they produced. Not in ten years at the mine.

Image: The moment that makes all the effort worth it - the pour

But then, this is a business. Seeing bars of gold isn't what pays the bill. What matters is shifting dirt and creating value.

The depths of Nevada might seem like an odd place to come to discover more about Brexit, but I travelled 5,000 miles to witness the beginning of another journey.

For what I'm most interested in is what happens to those gold bars when they leave the refinery. It is the start of a voyage with major consequences for our picture of the UK economy.

Image: After being heated to 1,000 degrees, the gold is poured into moulds

Though they're glistening by the time they leave Cortez, those gold bars are still only about 95% pure. So they're shipped, or rather flown, to Switzerland, where you'll find most of the world's major refineries.

They're melted down again and refined until they're more than 99% pure, and then they're flown to London. Why? Because it turns out Britain is the world capital for the trade of physical gold.

The chances are if a bar of gold is to be bought, sold or stored, it will happen in London. The world's accepted standard for bullion production - London Good Delivery - is, as the name would suggest, a British hallmark.

Be it historical accident or the result of long-term strategy, London is at the very heart of the gold trade.

Image: The bars are around 95% pure gold when they leave the mine

In some senses this is rather odd, given Britain has no major gold mines. It has only one surviving refinery, mostly melting down scrap metal.

The vast majority of the world's gold is, of course, mined elsewhere: in the US, South Africa, Australia and elsewhere.

Image: Baird, the last refinery in London

But sitting underneath the ground in warehouses inside the M25 are vaults containing well over half a million bars of bullion, worth a grand total of around $300bn (£223bn) - roughly the equivalent of £9,000 for every household in the country.

Unfortunately for us, this gold, much of which sits inside the Bank of England's vaults, isn't owned by the British people or the Government or the Bank itself.

Image: The bars waiting to be cleaned

Instead, it is stored on behalf of other central banks, financial groups and wealthy investors. In much the same way as the Square Mile has long been the world's favourite place to invest your money, it has also long been the world's favourite place to store gold.

When it comes to value added, gold storage and transportation comes a very distant second to the production of the actual metal. For all the allure of the glistening metal and the conspiracy theories that surround it, much of this activity is actually deeply unsexy.

Moving and storing a gold bar is really just a logistics job, albeit with slightly higher insurance premiums. And such is the secrecy and security in the trade that most of the participants would rather you didn't realise it was going on at all.

We spent months attempting to secure access to a gold vault, so we could see the next stop in the journey of our gold bars from the US - but with little luck. It turns out no-one in the gold business has much interest in being on television.

Image: One of these is worth nearly twice an average UK home

When, eventually, we persuaded one vault to let us onto their premises, we had to sign waivers promising never to divulge its location.

We had to agree not to film anything that might give the game away, no glimpses of identifying features, no shots of the security cameras or gates that surround the installation.

But, after all that, eventually I got to handle some gold bars, fresh from the vault.

There are three things that strike you when you lay your hands on a London gold bar, all 12 and a half kilos of it. The first is how mesmerising they are.

Image: The rocks are mixed with cyanide solution

There is a touch of Midas in us all - even the least acquisitive of us. The second is how heavy they are. There are few things you'll ever touch which pack so much weight into such a small size.

The third is getting your head round the sheer value. A single gold bar is, at today's prices, worth just over £400,000. Something about the size of a pencil case, and it's worth nearly twice the average UK home.

Which brings us back to London's role as the global hub for this stuff. On any given day there will be bars of gold passing in and out of Heathrow Airport, mostly in the bottom of commercial flights - coming in from the mines near Johannesburg or going out to the refineries near Zurich.

In much the same way as Heathrow is seen as a hub for flights, London is seen as the world's central point for physical bullion.

There are few things you'll ever touch which pack so much weight into such a small size

But because every gold bar is worth so much, it doesn't take many of them crossing in and out of the country to show up in the national statistics. Although it is not a producer, Britain is duty-bound to record the flow of gold in and out of the country in its trade figures.

This has given rise to some rather odd statistical anomalies.

In July, Britain's gold exports were worth more than any other physical export. More than motor vehicles, more than engines, more than pharmaceuticals or crude oil. In fact, gold accounted for more than one in ten pounds of UK exports that month.

And while the flows are volatile, this is hardly unusual. Over the past five years, gold was Britain's biggest export after cars - bigger than any other physical export.

Image: Drilling inside the mine

Occasionally, the statistical impact of the precious metal - little understood even by most UK economists - is so great that it can even affect our picture of overall economic growth.

Consider what happened last year, in the months following the referendum vote.

When economists spotted a sudden jump in Britain's exports, many interpreted it as a vote of confidence in the post-Brexit economy - or, at the very least, a boost from the weaker pound.

As it turns out, the majority of that sudden uptick was accounted for, single-handedly, by gold.

The days surrounding the EU referendum saw a massive surge in sales of bullion, as investors took fright at the changing political weather in Britain and loaded up on the safe haven investment.

A similar thing happened after President Trump's election.

Then, at the end of the year, some of those foreign investors shifted some of their gold out of London and brought it back home, mostly to China.

In other words, far from being a sign of confidence in Brexit Britain, this was quite the opposite - evidence that investors were getting jittery.

How gold distorts Britain's trade figures

But since London is where you come to buy the world's safe haven investment, and since statistical conventions stipulate that gold is marked down as a UK export when it leaves the country, this flight to safety showed up as a sudden jump in UK trade.

Actually, the export spike was so big it also pushed up initial estimates of gross domestic product in the final quarter of last year.

And here's the thing: since the majority of the bars go to Switzerland (to be melted down from heavy London gold delivery bars into the smaller kilo bars Asian investors prefer) and to China and India, this all shows up as non-EU exports. The upshot is that Britain's exports outside the EU look far bigger than they really are.

This raises doubts over one of the few Brexit claims which has yet to be challenged - that Britain now exports far more outside the EU than inside.

The official trade figures produced by HM Revenue & Customs show that over the past five years the EU's share of Britain's exports has dropped to 46%.

But strip gold out of the statistics and the EU's share is still 50%. Falling, yes, but not quite as fast as the official numbers might have you believe.

There are a few provisos: for one thing, these numbers don't include services trade - Britain's real speciality, particularly with non-European partners.

It means detonating hundreds of thousands of pounds of explosives

Even so, when you exclude gold from the overall trade balance (goods and services) - a tricky operation since the numbers are fiddly and not altogether comparable - a similar thing happens: the share going to the EU rises from about 45% to 47%.

The lesson is clear: that while Britain remains a dynamic trading nation, it is actually considerably more reliant on trade with Europe than the official numbers suggest.

All of this thanks to some gold bars passing through town - the butterfly effect of some rocks being shovelled in a mine on the other side of the world.

Yet that is the nature of the global economy into which Britain is plugged. In due course, the statisticians will find a way to adjust for these flows of gold. Those I've spoken to at the Office for National Statistics are well aware of the problem and are working on it.

In the meantime, this will go down as one of the more unlikely unreliable statistics of the referendum campaign. And a reminder of yet another industry where Britain leads the world. We may not make the gold, but it transpires there is no-one out there more renowned for looking after it.

:: Beneath the sound and the fury of the Brexit negotiations, what's actually going on ? "Brexit Forensics" is a regular series of stories from our top specialist correspondents, revealing the issues and the themes behind the headlines that will really determine the course of the Brexit revolution.