What are these large, empty towers made of connected metal girders, bars or pipes in London?

I saw one on the Greenwich Peninsula next to the O2 and another one while leaving Paddington towards Slough.
Courtesy: urban75.org
Best Answer
The technology is called a Floating Head Tank. The idea is that the tank is sealed around the bottom by two walls with water in between. As gas is pumped in, the tank rises. The weight of the tank keeps a constant gas pressure. Typically, the tanks would gradually fill overnight meaning a large capacity was available locally for daytime use yet with a small pump capacity.
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More answers regarding what are these large, empty towers made of connected metal girders, bars or pipes in London?
Answer 2
They're gas holders or gasometers - they're used in several cities around the world for storing gas. See Wikipedia for more information.
Answer 3
Tom Scott made an episode of "Things You Might Not Know" about these gasometers.
Answer 4
Gasometers, which store natural or town gas.
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