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

are tiny.

This is not something to worry about.

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

is spending a lot of money fixing them up.

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

a bit of pavement just goes... boomf.

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

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

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

with the help of a friendly pyrotechnician.

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

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

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

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

and over the course of decades

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

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

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

But an electrical fault won't make an explosion.

If electricity goes somewhere it shouldn't,

then the circuit breakers that control the grid will detect that

and shut it off right away.

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

Including gas pipes.

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

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

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

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

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

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

and something to make an electrical spark.

That's all you need for an explosion.

Fire in the hole.

"This looks devastating in slow motion.

"The burst of flame, the flying manhole cover,

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

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

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

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

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

Fire in the hole.

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

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