We like to tell ourselves that it's easy to distinguish between the natural and the artificial, but they have a knack for fooling us. When European colonists traveled through the patchwork of forests and meadows of New England, they thought they were exploring primeval nature. In fact, Native Americans had been tending it carefully with fires for centuries. When the Viking probe snapped a fuzzy picture of a mountain on Mars in 1976, some people were sure it showed a giant face carved by Martians. When another probe took a sharper picture in 2001, all trace of the face had vanished.
"All trace"? Hardly.
Rectilinear framing mesa? Still there.
And while "some people" were indeed "sure" that the feature was created by Martians, a lot of others were -- and are -- rightfully agnostic.