The Bible says clearly that every knew shall bow and every tongue confess that Jesus is LORD on the earth, in the heavens and beneath the earth. The Bible says clearly in Genesis that it is the story of the generations on the earth and in the heavens. Generations means mortals.
Thus, we have life on the planet, life on other planets and, more importantly, life inside the earth. So, do not be surprised when you discover there is a portal to inner earth in Antarctica. But, be advised that certain very bad angels are imprisoned in Tartarus which is as far beneath the earth as the heavens are above the earth, and it has inhabitants.