From the always-wonderful Atlas Obscura...
Carting Lane Sewer Lamp
The pretty iron lamp is a reminder of the architecture underneath London's streets.
Just off one of
London’s bustling streets, there’s a special streetlight that signposts the warren of waste pipes winding beneath the city.
By the end of the 19th century, London was trying to shed its reputation as a stinky cesspool. For years, locals had slogged through streets strewn with human waste. Now, it had an expansive sewer network that shuttled these excretions around. Trouble was, that matrix was smelly, too—and full of gas. Engineers proposed various ideas for ventilating the sewers, and one of those schemes included lamps that would burn off the gases from the underground world while illuminating the streets above....
https://www.atlasobscura.com/places/carting-lane-sewer-gas-lamp