The catastrophic collapse of a dam at a mine in Brazil has exposed a darker side of an industry that the world depends on.
At nearly 800 sites across the country and thousands more around the world, dams contain huge loads of mining waste.
One British scientist, Dr Stephen Edwards of UCL, has warned that “we are sitting on a time bomb”.
He told BBC News that further disasters were inevitable.
Over the last few days in the heart of Brazil’s mining belt, I’ve been investigating two very different sites where the risks of massive damage seem plausible.
One is a vast lake of sludge perched high above a nervous community; the other is an abandoned gold mine at risk of leaking poisons.
Why are there dumps of mining waste?
The answer begins with the world’s growing demand for metals such as steel which is used to make everything from buildings to ships to cars.
The key ingredient of steel is iron ore which is extracted from the ground in huge mines – Brazil is one of the world’s largest producers.
The ore is broken up, and because only a small fraction of the rock is actually iron, the vast majority is unwanted, a by-product that’s thrown away.
And the cheapest way to dispose of these remains is to create what’s called a “tailings pond” – a rather genteel term for a dumping-ground sealed with a dam.
Are these dumps easy to find?
Not always. The mining companies tend to tuck them away in valleys in the hills.
We used satellite mapping to find one of the largest tailings ponds in Brazil, a site called Maravilhas II, owned by the mining giant Vale which also owns the dam that failed last month.
Stretching as far as the eye can see is what looks like a sea of red mud, streaked with swirls of black and grey, a mass of sludge.
It was just this kind of material, as heavy as wet cement, that escaped in a torrent at Brumadinho killing, 169 people and leaving nearly 200 more still missing.
The waste is held back by a dam that’s constructed in a way unique to the mining industry.
Instead of using steel and concrete, as is typical in dams at reservoirs holding water, mining barriers are made of mining waste itself, band upon band of compacted sludge.
In the case of Maravilhas II, the dam stands an astonishing 90m high, a towering grass-covered wall raised in layers over the past 20 years.
What are the dangers?
According to Dr Stephen Edwards of University College London’s Hazard Centre, there are possibly thousands of dams of this kind around the world.
“We are sitting on a time bomb. The big problem is that we don’t know which offer the greatest threat and where they are situated.”
Dr Edwards says that although the number of failures is decreasing, the size of the sites is increasing so the impacts of collapse are becoming greater.
“Many tailings facilities are the biggest structures on the planet and that trend will increase and, if we are getting bigger facilities, and some of those might fail, then we are going to end up with bigger disasters.”
So what’s it like living in the shadow of a dam?
For Cristiani Magalhaes, whose home is a few hundred metres of the massive Maravilhas II dam, it’s a nightmare.
“I don’t sleep anymore since what happened in Brumadinho,” she tells me. “It was a warning: is the same going to happen here?”
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