DANIEL SAMBRAUS / SCIENCE PHOTO LIBRARY DANIEL SAMBRAUS / SCIENCE PHOTO LIBRARY
Rooftop gas tank on top of an apartment block. This tank contains natural gas (mostly methane), which is used as a household fuel in appliances such as cookers and in central heating systems. The gas is under pressure in the tank, and a valve (blue) releases it into the pipes (yellow) that carry it into the apartments below. Photographed in August 2008, in Mexico City, Mexico.
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