Below you can find the Once thought to be the work of an advanced civilization these features are now known to be an optical illusion Jeopardy Words Answer. The answer to Once thought to be the work of an advanced civilization these features are now known to be an optical illusion has 6 letters. This clue belongs to the The red planet category and is part of Jeopardy Word Level 500. If you scroll near the bottom you will find other clues and answers part of level 500