a lot of people have got in touch to tell me that in Warsaw, in Poland,

the tap water quality is tested and controlled by clams.

They said that the city constantly runs a sample of its tap water past some mollusks.

And those are filter feeders, so if there's a contaminant in the water, they will instinctively close.

Sensors attached to them will detect that and raise the alarm.

And I didn't believe it.

It seemed too weird to me.

And all the articles that I could find about it seemed dodgy, somehow.

They just set off red flags in the research parts of my brain.

There is a very arty Polish documentary about the system,

and a lot of sites repost that film's claims without even mentioning the source,

just as a "look at this funny thing off in Poland!"

It felt like it could be a misinterpreted or mistranslated joke,

or even a deliberate hoax by a filmmaker that got out of control,

and that the English speaking world was just believing it because it was quirky and harmless enough.

There have been plenty of fake things like that spread about in the past, accidentally or otherwise.

Or maybe the clam-sensors did exist, but it was a brief experiment that never went anywhere.

Well, I'm now here in Warsaw, on a famous building in the middle of the river,

with a Polish speaking team showing me around.

There is a lot of stuff we are not allowed to film here for security reasons.

Inside here is critical infrastructure.

Plus, it is far too noisy down there for me to talk,

but I'm happy to report that yes, the clams are real.

They actually do protect Warsaw,

but the story's a little more complicated than that.

Warsaw's water source is surface water, the water of the Vistula River.

If it turns out that there's ever a toxic substance in the water,

we must not allow that contaminated water to reach our residents.

So therefore, we would start a special procedure,

and our laboratories would carry out a number of tests,

to find out what is the cause of the water contamination.

And we would have about two days to do that.

The alarm has never sounded yet, but for water safety,

an early warning system is very important.

The important thing is: those clams aren't the only protection.

They're not even the main part.

This is still a modern water treatment system,

with 21st century sensors and detectors to keep everyone safe.

This is part of a defense in depth, one part of an early warning system,

one more alarm that can go off,

like an airport that uses explosive sniffing dogs but also electronic detectors.

These clams are caught from a very clean lake.

Then they got to our laboratory where they spend about two weeks acclimatizing.

At the same time, although it sound strange, they're also calibrated.

What angle of opening is 100% and what is zero?

It's very important that we're able to interpret the result later.

Eight individuals go to this aquarium.

And these clams are attached to pedestals to which there's also attached a probe,

receiving signals from the sensor that's mounted on the shell of each clam.

These sensors are fixed with a special glue, which is absolutely safe for animals.

Three months later, the clams are returned to their natural habitat.

We know that this is not a stressful situation for them.

We take care of our colleagues here!

How do we know if there's a problem, if something's happening to the water?

If, at the same time, six out of eight individuals close their shells for more than four minutes,

and at the same time, average closing percentage of all the shells falls below 25%.

We can see that most of these clams are very active now.

The average opening percentage, across all the clams, is 72%.

So, the water is safe.

The technical term for this is "biomonitoring,"

and it turns out from more research that this isn't the only place that does it.

There are other water companies in Poland that have similar setups,

plus one in Minnesota, USA, and probably more that I haven't found.

At which point I think I can say that: yes, the red flag in my head was wrong.

The clams in this building really do help protect Warsaw.