I think that the most fearsome attribute of these awesome creatures is their silence and the fact that one never sees them—nothing but those baleful eyes glaring unblinkingly out of the dark void behind.
1913 January–May, Edgar Rice Burroughs, “The Gods of Mars”, in The All-Story, New York, N.Y.: Frank A. Munsey Co.; republished as “The Eyes in the Dark”, in The Gods of Mars, Chicago, Ill.: A[lexander] C[aldwell] McClurg & Co., 1918 September, page 227