The scourge of spellers, silent letters are often ... that way in English. This word (from Greek kthon, meaning "earth"), tends to lose its initial "k" sound and ends up sounding like thonic.

Silent letters are the ghosts of pronunciations past. The word 'knight', with its silent 'k', and silent 'gh', is cognate with the German word for servant, 'knecht', where every letter is pronounced.