The roar of the Zambezi is deafening as millions of gallons of water crash over rocks and tumble down rapids.

But there's another sound cutting through the trees of the Zambian bush - the unmistakable high-pitched whine of a bitcoin mine.

"It's the sound of money!" says a smiling Phillip Walton as he surveys the shipping container with 120 computers busily crunching through complex calculations that verify bitcoin transactions.

In exchange they are automatically rewarded bitcoin by the network.

We're in the far north-western tip of Zambia near the border with the DRC, and of all the bitcoin mines I've visited - this one is the strangest.

Water and electronic equipment don't usually mix well but it's precisely the proximity to the river that's drawn bitcoiners here.

Phillip's mine is plugged directly into a hydro-electric power plant that channels some of the Zambezi's torrent through enormous turbines to generate continuous, clean electricity.

More importantly for bitcoin mining - it's cheap.

So cheap it made business sense for Phillip's Kenya-based company Gridless to drag its shipping container full of delicate bitcoin mining computers across bumpy narrow roads 14 hours from the nearest major city to set up here.

Each machine makes about $5 (£3.90) a day. More if the price of coins is high, less if it drops.

Occasionally Phillip glances down at his smart watch - the home screen showing the ever-changing squiggly line of bitcoin's dollar value.

At the moment it's around $80,000 a coin, but Phillip says they can make a profit even when the value of the bitcoin goes low thanks to the cheap electricity on the site and the partnership they have with the energy company.

"We recognised that in order to get better mining economics we needed to partner with the power company here and give them a revenue share. And so the reason we're willing to come out here somewhere so remote is it allows us to effectively get cheaper power," he says.

Zengamina hydro-power plant is huge but technically it's a mini grid - a standalone island of power for the local community.

It was built in the early 2000s thanks to $3m raised from charity donations.

British-Zambian Daniel Rea runs the site after his missionary family led the building project, primarily to power the local hospital.

Now it provides power for around 15,000 people in the local area but the project hasn't been able to make ends meet because of slow take-up from the community.

Allowing the bitcoiners to set up shop here has been transformational to the business.

"Every day we were wasting over half of the energy we could generate which also meant we're not earning from that to meet our operating expenses. We needed a major user of power in the area and that's where the game-changing partnership with Gridless came in," Daniel says.

The bitcoin mine now accounts for around 30% of the plant's revenue allowing them to keep the prices down for the local town.

Bitcoin and its economics are of course far from the minds of the people in Zengamina.

The town itself is a few miles from the plant and comprises not much more than a few dozen shed-like buildings peppering a cross roads.

Only one shop has a fridge and a dozen kids crowd around a communal computer taking turns to choose a song to blare out, causing adults to wince as they go about their day.

Although the hydro-electric plant came online in 2007, it took a few more years to connect it to the local town, and then more time to connect individual homes and businesses.

So, some people like barber Damian are still enjoying the novelty of getting wired up only a year and a half ago.

"Until I got power I had nothing and couldn't do anything. When I got power I bought everything at the same time."

He's not joking. At night his tiny barber shop is a beacon of power with a TV playing music videos, strings of Christmas lights and the buzz of his hair clipper. Like moths, young people hang out in his barber shop like a youth hostel.

"Getting power has changed my life," he smiles. "The money I'm earning now from the barbershop is helping me pay for school fees again."

Embracing electricity is very much a business decision for Damian. At home he shares one light bulb between the two rooms that make up the small house.

Elsewhere in the town sisters Tumba and Lucy Machayi sit on the crossroads watching the world go by.

Like many young people, they're glued to their phones.

"Before the town got power, it was basically just the bush," says Lucy.

The little electricity they had used to come from small solar panels, they say.

"No fridge, no TV, no mobile phone network," says Tumba.

"Electricity completely changed the lives of people here," Lucy adds.

"We can charge our phones, we have network. We can communicate with each other."

Not many people here know or care about the bitcoin mine that's played a part in helping the hydro-plant keep things going.

But soon they'll watch as that container once again rattles its way through the town on its way to another location.

Zengamina Hydro has secured a large investment to help them expand to more villages and join up to the national grid.

Soon the excess energy the mine was harvesting will be sold back to the national grid and mining bitcoin will no longer be profitable at Zengamina.

Phillip and team are sanguine about this and insist this is good news. They will have had a successful few years here and ultimately they are happy to have helped Zengamina. And made a tidy profit in bitcoin of course.

The company says there are plenty of places with so-called stranded energy that they can plonk their bitcoin mine next to.

Gridless already has six sites like this in three different African countries.

North of Zengamina another bitcoin mine slurps up excess energy from a hydro-electric plant run by Virunga National Park in the Congo. It's helping to fund conservation projects, the park says.

But Gridless now plans an ambitious next move - to build their own hydro-plants from scratch to mine for bitcoin and bring electricity to rural areas.

The company's co-founder Janet Maingi says the company is busy raising tens of millions of dollars for the project.

They're focusing on so-called run-of-river hydroelectric models like at Zengamina and the continent has an abundance of "untapped hydro potential" she says.

"A consumer-driven, adaptive energy model is essential for scalable, affordable, and sustainable energy access that meets the needs of African communities," she explains.

The company is not a charity and believes that ensuring long-term economic viability for developers and investors can only be done through bitcoin.

Finding locations for a new plant or to tap into existing ones is the easy part though.

The company still faces resistance from some authorities and companies which see bitcoin as an energy-greedy and selfish use of electricity that might otherwise be used by rural people.

But the company insists that the incentive is always to sell to the highest buyer and that will always, they say, be the local community.

History tells us that without incentives or rules in place, bitcoin mining at scale can put strain on public energy grids. In Kazakhstan in 2020-2021 a mining boom increased energy usage in the country by 7% before the government clamped down and clipped the wings of the burgeoning industry.

In the US - bitcoin mining's new mecca - conflicts between miners, locals and residents have been common when electricity is in high demand.

Authorities have created agreements with some mining giants to ensure that they power down their warehouses chock full of computers at times when the grid needs balancing.

For example, Greenidge gas power plant in New York which was renovated to mine bitcoin was mandated to power down mining in January to supply electricity to the grid during a cold snap.

Agreements like these will need to be widespread if President Donald Trump's ambition for bitcoin to be "mined, minted and made in the USA" is to be achieved.

The environmental impact of the industry is also a major concern. It's estimated that bitcoin mining uses as much energy as a small country like Poland.

But according to researchers at Cambridge University which does annual estimates on bitcoin's energy usage, there is a shift taking place to a more sustainable energy mix.

Set ups like this Zengamina are a tiny part of the overall mining picture.

But they are also a rare example of a controversial industry creating much more than just digital coins.