This total disappearance of these marvellously protected giants, which seemed capable of defending themselves against any and all kinds of enemies that might have arisen, is one of the strangest and most unsolvable problems of science.
"The Human Side of Animals"
Royal Dixon
The Tepoktans loved speculating about the unsolvable.
"Exile"
Horace Brown Fyfe
All we can say is, that the problem of life in Mercury belongs to the class of unsolved, and perhaps unsolvable, mysteries.
"The Story of the Heavens"
Robert Stawell Ball