Mill Ends Park in downtown Portland, Oregon, is a small park that was created on St. Pratrick's Day, 1948, to be a colony for leprechauns and a location for snail races. It is the smallest park in the world, according to the Guinness Book of Records, which first granted it this recognition in 1971. The "park" is a circle two feet (610 mm) across in a traffic median which in 1948 was intended to be the site for a light pole.
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Wikipedia article:
http://en.wikipedia.org/wiki/Mill_Ends_ParkCategory:
world park smallest