The cave is the lower space where the Dimension Card Company was previously located...
This should originally be a large hollow mountain belly, able to accommodate animal experiments and personnel training, while also housing thousands of cattle and sheep to shelter from the winter cold, and providing food and animal feed for the Dimension Card Company people.
Mountain Eagle used bombs to destroy Dimension Card Company's laboratory, and the explosion in the upper space caused an internal collapse of the mountain, burying a large portion of the massive lower space with the collapsed mountain.
In the past period, after those so-called protection organizations diligently excavated and explored, the location where Mountain Eagle and his team are now has finally seen the light of day again.
