The observation that began the 19th and early 20th century fascination with Mars as a place for life was:

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Schiaparelli's seeing what seemed to him to be long straight features on the red planet, which he called "canali," began the fascination with Mars as a possible place for life. This was later misinterpreted as "canals" and led to theories about intelligent life on Mars, most notably by Percival Lowell.

The Observation That Began the 19th and Early 20th Century