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The city of Tokyo is built around and over rivers.
When you travel around here, it is striking just how often you cross water.
And from up above, you can see this whole area is flat, low-lying coastal land.
Some parts of Tokyo even sit below sea level.
They've sunk because of decades of pumping out groundwater.
The two and a half million people in those low-lying areas
are protected by a network of flood gates and aging levees,
which'll be fine unless sea levels rise.
But even inland,
a huge part of this megacity is built in river basins and flood plains.
And as the climate breaks down,
rain storms here are getting more intense and more frequent.
If you want to prevent river flooding,
or drain neighbourhoods that are starting to flood:
where do you put the water?
If you pump it out of that neighbourhood,
you're likely just moving the flood next door.
The best plan would, in theory,
be to pump the floodwater all the way to the ocean,
or at least to a tidal river that can deal with it.
But for that, you would need tunnels and buffer tanks on an almost unimaginable scale.
Now, despite what some breathless blog posts might claim,
the whole metropolis doesn’t have that.
But there is one place, about an hour north of the centre of Tokyo, that does.
This is the Metropolitan Area Outer Underground Discharge Channel,
or at least, one part of it: the holding tank.
It is almost impossible to convey the scale of this on camera.
If the Otoshifurutone and Naka rivers start to flood,
this is where the water will drain to.
Actually, where the water is sort-of still draining to.
There was a massive rainstorm about two days ago,
and there is still water in the system.
I am not allowed down on the floor, just in case.
This tank, and the kilometres of tunnels and all the silos that bring water to it,
cost nearly a quarter of a trillion yen,
that's about two billion US dollars, at the end of the 20th century.
That was when the Japanese government was spending money
to finance enormous infrastructure projects like this,
to try and stimulate the economy.
Which, for long-term security, is great:
but it took a national recession to make this happen.
A plan like this only works if a government wants to spend a colossal amount of money,
and in the 21st century,
there aren't many governments willing to do that.
Thank you to all the team here for letting me down in the tank,
and up on these catwalks. You can find out more about them
and their work at the link in the description.
[echoes continue and fade]
That goes for a long time.