A century ago this was called the Richest Hill on Earth.

It was the Anaconda Copper Mine in Butte, Montana.

It's not rich anymore, and it's not much of a hill.

In the 1950s, the mine changed from digging underground tunnels

that followed particularly rich veins of copper,

to just excavating everything,

turning the hill into an open pit

on a scale that is difficult to convey on camera.

There were whole neighbourhoods destroyed to make way for this.

It is a mile wide, and so deep that you could fit

the new One World Trade Center standing upright in it.

Or at least you could,

if the pit wasn't halfway filled with toxic dilute acid.

It kills any birds that land in there for too long.

In 2016, thousands of geese died in one night

after a snowstorm forced them down there.

The team here let me get a little bit closer than the tourists do.

- Some studies were done to try to figure out

how long a bird can withstand sitting on the pit water

and those studies found that it's up to 18 hours.

Six to seven thousand birds come through

and utilise the Berkeley Pit as a resting stop per year.

We're on the overlap of two major flyways

so it's to be expected. [deep boom]

- Can you tell me what that was?

- So that's a propane cannon, we have them on timers

and they run 24 hours a day, seven days a week.

Hopefully, deter the birds from ever considering

landing on the water of the Berkeley Pit to begin with.

If the birds land, we have a crew of personnel

that observes from this shack on the hour.

They log them and then attempt to haze them off.

The go-to is high-powered rifles, you have that ability to hit very close

to the birds from a very far distance away.

And it's not so much the boom as you would expect that scares them

but it's the projectile hitting the water,

creating a large splash, and that's what usually does it.

And in the case that we know a large flock of birds is coming,

we can have continual fireworks

to deter the birds from entering the pit area.

We get almost all of them, we are over 99% effective.

- There's around 10,000 miles of mine shafts and tunnels

under the city of Butte.

That's not an exaggeration. 10,000 miles.

Groundwater and rainwater had to be constantly pumped out.

Nearby pumps moved somewhere around 300 litres a second.

But when this pit shut down in 1982, so did the pumps,

and water started to flood the old tunnels and the pit.

The rock here is full of iron pyrite.

Combine that with oxygen and water and as it rusts, you get sulfuric acid.

That in turn, breaks down other metals and minerals in the rock.

And the result is that the water down there

is so acidic and so contaminated with toxins

that nothing except a few hardy bugs can live in it.

The deep groundwater here is utterly contaminated

but it's a closed system.

The sulfuric acid can't get in or out, for now,

because there's one more catch.

Water is still flowing into the pit

and if it gets above a certain level,

then the nasty stuff will start to leak out.

- So down here we're at the Berkeley Pit pump station.

Right now the Berkeley Pit is the sink or the sump,

like a hydraulic control for the system.

So what we're doing here is we're pumping, treating

and discharging water from the system.

And this includes bringing Berkeley Pit water on line

and treating it through an existing water treatment plant.

If the Berkeley Pit is not maintained,

the protective water level would be reached in July of 2023.

pH at one time was as low as about 2.2 or 2.3.

Currently up to about 4.1.

The water that we'll be discharging from the site

will be clean water, about 7 million gallons per day.

2100 gallons per minute.

Since 1982, the water level has been increasing,

in the neighborhood of about six to seven feet per year.

Later this year would be the first time since 1982

that the pit water level will be maintained and held steady.

- Tourists go here because, well, tourists will go anywhere interesting

and this is interesting.

In 2007, Dr Phaedra Pezzullo of the University of Colorado, Boulder,

coined the term 'toxic tourism'.

The fact that people will come to gawp or be shocked

or just to take a selfie, and say that they saw

the mile-wide pool of toxic waste.

And there are much worse ways to raise awareness

and tell the world, hey, this is pretty screwed up

and we need to do something about it.

A $3 admission fee is, well, it's capitalism.

In the United States, a country that codified the idea of the roadside attraction,

it almost seems normal to pay for a ticket

to see the mile-wide, incredibly toxic pit.

Thanks to all the team from Montana Resources

and the Berkeley Pit who helped make this video happen.

You can find out more about them by pulling down the description.