Though I distinctly remember reading in an encyclopedia from the '70s that certain large dark regions on the Martian surface were believed to be thick vegetation (: I'm not kidding!

From [1]: "As recently as the early 1960s, it still seemed possible to a few astronomers that the dark regions had some kind of plant life because they seemed to darken each summer as if plants were growing in response to sunlight."

[1] https://www.jpl.nasa.gov/images/pia02362-the-dark-surfaces-o...