Gas (the domestic gas that you use for cooking, not petroleum) used to be supplied in the UK from centralised gas works, I'm not sure of the precise method, but I believe that it came from heat treating coal. The gas was then stored in huge metal cylinders called gasometers. These cylinders were capable of expanding and were surrounded by the metal work you see in the picture.

We now use natural gas so the gasometers are redundant, but in many places the metal work is still in place, as here.