Beneath the massive blue-lit cavern of K'n-yan are the red-lit caverns of Yoth where the serpent folk of yig moved their civilization in ancient times. All that remains now are ruins and serpent-tech... and a way to even deeper lightless N'kai, home of Tsathoggua.
https://dysonlogos.blog/2022/10/07/release-the-kraken-on-ruined-cities-of-red-litten-yoth/