Koneczny, Feliks (1962) On the Plurality of Civilizations, Posthumous English translation by Polonica Publications, London ASIN B0000CLABJ. Originally published in Polish, O Wielości Cywilizacyj, Gebethner & Wolff, Kraków 1935.
Schmidt, Gavin A.; Frank, Adam (10 April 2018). "The Silurian Hypothesis: Would it be possible to detect an industrial civilization in the geological record?". arXiv:1804.03748 [astro-ph.EP].