I am building a science fiction universe, and I am currently thinking of many different unique sentient alien species that would populate the galaxy.
I have thought of a rocky planet with Earth level gravity which has a highly elliptical orbit that causes its tempetures to have massive fluctuations between hot and cold. These extremes make life inhospitable on the surface. However underneath the surface there are large pockets of space that are filled with water up to a certain point and then open air. Imagine massive deep lakes with large amounts of land surrounding them.
The only input of energy into these areas are very large numbers of hydrothermal vents similar to what we see at the bottom of the Earth's oceans. This planet has a very active core causing high amounts of energy output through these vents. My main goal is to have a sentient, humanoid creature that creates a decently complex society within the many pockets.
Edit: I have added ventilation shafts that serve to dissipate heat out of the pockets and assumed that there is a balance between the amount of heat produced by the hydrothermal vents and the heat dissipated out of the pockets.
The issue though is the lack of sunlight, and lack of an oxygen based atmosphere. Assuming that the pocket remains at a stable temperature and does not blow up, what kind of ecosystem would arise from this environment, and what gases would these creatures breathe to live?