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A POTD is an acronym for "Photo of the Day", "Picture of the Day", "Post of the Day" or "Poll of the Day". It is a term most commonly used by online digital photography communities as an honor to outstanding photos that are selected daily from among that community's membership. They can either be tagged as POTD or displayed in a special page on that site...

Those gasometers were built in Vienna, Austria in 1896. Now they have been renovated and are used as environmentally friendly living spaces. A gas holder (commonly known as a gasometer, sometimes also gas bell, though that term applies to the gas holding envelope alone) is a large container where natural gas or town gas is stored near atmospheric pressure at ambient temperatures...