Why are so many sports venues circular, or nearly circular? Circles are an incredibly efficient shape. They allow builders to fit many spectators into a fairly small space. Since there are no corners, every seat has a decent view of the action.
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The ancient Greeks and Romans built some of the world’s first circular stadiums and arenas. In fact, the word “stadium” comes from the name of a Greek measurement, and “arena" comes from the Latin word for sand. (The Romans covered the floor of the Colosseum with sand. When gladiators fought to the death, the sand absorbed their blood and kept the floor clean.)
The Colosseum in Rome was not built as a perfect circle but rather as an ellipse. However, it was still a more efficient design than a rectangular stadium would have been. The amphitheater could seat 80,000 people and had a removable floor. Sometimes, the Emperor would flood the Colosseum so that spectators could watch naval battles. Today, outdoor stadiums routinely hold more than 100,000 people.
See for yourself: http://www.youtube.com/watch?v=YViOCl_YAh8
Check out the videos below for a glimpse into more famous stadiums.