Why do London's manholes keep exploding?

Every month or so, somewhere in London, a manhole explodes. It's so common that it doesn't make the news unless it's spectacular or someone gets injured. Here's why, complete with gratuitous pyrotechnics.

Filmed by Perspective Pictures
Pyrotechnics by Paul at Darkstar Pyrotechnics

News articles:
https://www.telegraph.co.uk/finance/newsbysector/energy/11169183/Explosion-in-city-street.html
https://www.standard.co.uk/news/london/pavement-explodes-yards-from-pedestrians-and-cars-in-old-street-a3568676.html
https://www.express.co.uk/news/uk/819198/london-manhole-cover-explodes-old-street

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