Hello. My name is Brian McKinley. I beginning work on a hard(ish) science fiction comic titled Verdant Spiral. I thought this would be a good place to get feedback to make sure my ideas are plausible. All feedback is welcomed, and thank you all in advance. The first story takes place on an exomoon orbiting a gas giant around the white dwarf G29-38. The surface of the moon is frozen over like Europa or Enceladus. I was inspired by images of methane bubbles trapped in frozen lakes to imagine something similar happening on a massive scale. hydrothermal vents maintain a liquid ocean under a thick icy crust. Cavities are cut upward into the icy crust and gases are trapped there. Once there is an atmosphere trapped there an ecology develops around it. That ecosystem continues to evolve and intelligent life develops, builds a civilization, dies out, humans come exploring. So what I've been wondering is the formation of these sort of cavities that could fill up with atmosphere possible? What shape would they be likely to take? Could the cavities be open to the ocean at the bottom? Variations on shape I had thought of: The cavities are cone-shaped, big end up. Sort of like upside-down, inside-out island chains. The cavities are stacks of flattened bubbles like the photos that inspired the idea. The cavities are bell-shapped, big end down. If an opening to the ocean is plausible, then the idea with big end up is the descent through the cavity gets more and more claustrophobic and terminates in a small hot-spring area. With bell shaped I guess it would have to be a large lake or sea and the dramatic implications change.