Source of the post
I have no idea if this was already suggested or even possible but wouldn't it be cool to have cave systems throughout some planets? Gives that extra variety.
Welcome to the forum.
Your suggestion is in the TODO list of the developer as "Holes in the surface (caves, mines)". This means is in the plans for a future SpaceEngien release.
You can read the whole list here
Please, when asking for a new feature first take a moment to read the TODO list and second do a search. It is important to keep the forum clean and not full of similar suggestions. On the other side if you think this feature is important for a more in depth discussion consider making a new thread.