In the dead of night we've come to film a mafia at work.

We know the dangers. We've been told our lives could be at risk.

But this is the vision we must have to expose an extensive network of underworld criminals, known as the "sand mafia", which is destroying beaches and riverbeds across India.

Foreign Correspondent travelled to the drought-stricken Bundelkhand region in central India to film them at work, stealing sand — a resource that's now so valuable it's been dubbed "India's gold".

Sand is vital for India's booming construction industry, which is tipped be in the top three in the world in less than a decade. It employs over 35 million people and is valued at well over $126 billion per annum.

It's hard to move unnoticed through the primitive surrounding villages. As a foreign crew, we really stand out, but an element of surprise is vital for what we want to do. We retreat to a safe house, and wait for the go-ahead.

A sand dredging operation near Mahad. ( Foreign Correspondent )

Hours later as an evening quiet descends across the hills, we hear the tractors moving in to a nearby river bed. That's our cue to move too.

It's pitch black as we drive closer on a sandy, windy path. We're whispering, we're unsure.

Before we know it we see what we've come for. We scramble out of the vehicle and film as a tractor-load of sand drives past us. It's sand mafia men with their booty.

We need to get closer, but with sand everywhere we risk our car being bogged. We can't be left without a way to quickly escape. So we begin to walk in on foot. But as we approach we notice a shadow ahead. There is someone there — and it's too dangerous for us to keep going.

We've blown our cover now and the remaining tractors begin to leave at speed. One almost rolls over, careering around the bend as it tries to escape our camera.

It's time for us to go too, but the sand mafia want to make sure we won't come back. So they send an escort — a couple of men on a motorbike. They follow us for a while, but when they're sure we are heading out of the area, they take off and we manage to leave safely.

The Foreign Correspondent crew wait to get the go-ahead to chase the sand mafia. ( Foreign Correspondent )

Deadly and corrupt — India's 'largest scam ever'

The people at the heart of these illegal sand-mining operations can be incredibly dangerous, and will stop at nothing to get their hands on this valuable and diminishing resource.

Those who've tried to stop it have been beaten, maimed and in some cases murdered — accidental activists whose lives have been turned upside down by these criminals.

In the satellite city of Noida on New Delhi's outskirts, Akaash Chauhan is fighting for justice. His father, 52-year-old Paleram Chauhan, was shot dead as he fought to save communal village land from being completely stripped of sand.

Akaash Chauhan and his mother hold up photos of his murdered father and brother. ( Foreign Correspondent )

"When I reached the hospital and I saw my father's dead body, I have never been able to forget that sight. Even today, it flashes in front of me," Akaash says.

"My father's fight has become my fight.

"Sand mining is ongoing — my father was against it, I am against it and so is my family."

Sumaira Abdulali, a genteel unassuming woman, is one of India's foremost campaigners against illegal sand mining and the sand mafia.

She spends much of her time travelling around the country gathering data on how much illegal sand mining is taking place and how much money it generates. In the process she has been insulted and threatened.

Her public profile gives her a degree of protection now, but in the early days of her activism she was beaten and hospitalised when she tried to save Kihim beach, near Mumbai, from being stripped bare by sand gangs.

"It's probably the largest scam ever in our country," she says.

Powerful politicians and their business allies stand accused of allowing the illegal trade to thrive in return for generous financial kickbacks. But riverways and beaches are being destroyed — their eco-systems changed forever.

Foreign Correspondent also succeeds in capturing an illegal sand tractor operating in broad daylight. The workers shovel sand into the tractors by hand. They're from the lower rungs of the black market trade.

A sand mining tractor operating in broad daylight near Noida. ( Foreign Correspondent )

"I feel bad that I do this job but there's no other work I can do," the tractor driver said after being blocked by our car.

"I get a little extra money that is why I do it. Everybody does what they do for their stomachs."

Illegal sand mining is the dirty secret at the heart of India's booming economy. Conservatively worth $250 million a year, there's little political appetite to stop it.

The theft of sand occurs in a range of ways, from high tech dredging, to digging with bare hands, to free-diving.

On Mumbai's Thane river, local fishermen plunge to depths of 40 to 50 feet to gather sand from the riverbed.

With no safety apparatus and up to two minutes of air in their lungs, they fill a tin bucket with black muddy sand. It's an illegal act, but we have convinced them to allow us to film.

Incredibly, the men are often drunk. It's the only way they can calm their nerves to succeed in filling the bucket up to 200 times a day.

Sand miners on the Thane River near Mumbai, India ( ABC News: Foreign Correspondent )

Sumaira Abdulali regards these men as victims of the sand mafia, not criminals.

She is full of praise for her fellow activists because she knows they are mostly isolated and must act alone without the protection of the media spotlight, putting themselves and their families at risk.

"I think their bravery is really astounding," she says.

"Unlike me, they have no real way of reaching out and telling their stories when they start. So they must feel so strongly about the issues and their lands."

You can watch Foreign Correspondent: Line in the Sand on iView.