system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: I often hear about new planets being discovered with water beneath the surface, but rarely about planets with large oceans. Is subterranean water easier to detect or more likely to occur, and why?
gpt
Water only exist in a liquid state at a narrow range if atmospheric conditions.  

Many planets have surface temps far too hot due to proximity to s star or lack of protective atmosphere.  On these planets beneath a protective layer of crust is the only place liquid water can exist. 

 Flip that logic and there are planets with icey surfaces due to extreme low surface temps and atmospheric pressure conditions.  Those planets with an active core such as ours may allow liquid water deeper under the surface where the temperature is above freezing point.