Detail this asset from Unity Store: Original Link
This is a paid asset, but now you can download Ancient Caverns Free.
Ancient Caverns v1.0.1
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With Ancient Caverns you find a complete tool for your natural underground environment. It comes with over 180 prefabs including different big blocks to create custom rooms and also premade rooms to make it easier. Just add different details from rocks to stalactites, manmade stuff, candles, fungus, etc. Blocks are made to fit among them with a few clicks.
You can also use it as FPS or Top/Down environment since rooms are made of separate pieces so you can remove roofs whenever you want.
Check out [ASSET INFO] link above for extended info.