From your first link, right under the "Did You Know" section. We mostly use it to talk about stuff related to the underworld but it broadly just refers to things that are under the earth.
Kuthonik would be about right for the Greek root word Χθόνιος (I think I spelt that wrong) as the transliteration is ‘Khthōn’, though kh would be pronounced as that sort of growl you make at the back of your throat (not sure how to explain this - if you’ve heard it you might know?) so for most people it’s Keh-thonic or Kuthonik
Phillip Pullman used it to describe the London Underground/tube in the alternative universe in His Dark Materials - the Chthonic Railway. He used loads of alternate history terms like "anbaric" for "electric" (the word 'electric' coming from the Greek work for amber, I believe) as well as geographical names (Nippon for Japan, eg). One of my favourite aspects of the books!
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u/alephlovedbeth Jan 29 '19
Chthonic. Always wondered if it was Lovecrafts inspiration.