WEBVTT
Kind: captions
Language: en

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I want to be really clear:

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the odds of a manhole in a London street suddenly
exploding while you're next to it

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are tiny.

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This is not something to worry about.

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And the company that handles the
cables under London's streets

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is spending a lot of money fixing them up.

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But, yeah, about once or twice a month
on average, somewhere in London,

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a bit of pavement just goes... boomf.

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But no-one wants to license security camera
footage of actual pavement explosions

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to just someone on YouTube, and no-one wants
to be interviewed about it either.

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So instead, we are going to create our own
underground explosion right here

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with the help of a friendly pyrotechnician.

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And the easy answer to why London's pavements
keep exploding...

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well, it would be "electrical faults", and
that's what the news often blames.

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As seasons change, the ground is going to
get warmer and cooler,

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it's going to expand and contract,
rain and subsidence will move the soil,

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and over the course of decades

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that is going to steadily wear out
the 36,000km of electrical cables

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and 100,000 electrical junction boxes
that are under London.

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Most of those were laid decades ago.
It's a big city. Things will break.

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But an electrical fault won't make an explosion.

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If electricity goes somewhere it shouldn't,

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then the circuit breakers that
control the grid will detect that

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and shut it off right away.

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The trouble is that there's a lot of other
old stuff under London's streets.

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Including gas pipes.

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A steady, slow gas leak in a pipe might not
be detected for a while,

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and in that time the gas can spread a long
distance through narrow underground tunnels.

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So if you've got gas in a tight underground
space where it can't expand,

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thank you, and an electrical spark
to set it off...

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well, that's all we're doing here:

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under that cover there is a fuel source,
not enough room for it to expand,

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and something to make an electrical spark.

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That's all you need for an explosion.

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Fire in the hole.

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"This looks devastating in slow motion.

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"The burst of flame, the flying manhole cover,

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"at 1800 frames per second on a zoom lens
it looks like it could do serious damage.

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"But even this dramatised explosion, made
with the help of a pyrotechnician,

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"this worst-case-scenario...
is actually not that bad.

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"I tried to do the 'cool guys don't look at
explosions' shot,

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"and when you take away all this style..."

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Fire in the hole.

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"It'd hurt if you were standing on it, sure,

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"but like I said, it's not something to be
worried about."