I had a unique idea for an alien planet. The surface would be snow-capped mountains and glacial fields, it's cold and essentially devoid of most life. But just underneath the towering mountains would be this whole other world. A vast interconnected web of caves, tunnels, and hollows that essentially act as a planet-wide terrarium. There are even openings allowing sunlight to stream in allowing photosynthetic lichens to grow, forming the base of this world's food chain with creatures feeding on the lichens and creatures feeding on the creatures, feeding on the lichens. these openings also play a part in this planet's water cycle with water vapor evaporating through some of the openings like vents, only to freeze and fall back down as snow onto the mountains and glaciers, for the melted runoff to flow back into some of the openings starting the whole cycle over again.
But while this is an exciting idea for a setting, I am struggling to answer the question that interstellar explorers will undoubtedly ask when they find this place. How the heck did all this happen? So once again I am calling on the wisdom of the crowd to answer this question.
What scientifically plausible explanation, explains how this vast nexus of hollows, caves, and a thriving underworld on this alien planet come to be?