
Kailasa Temple
It's a mystery, how can such a massive place be built just for worship - there are many underground man-made caves over 300 km that serve as part of a temple or was it really an underground city with hopes for a better future far from the non-believers? Is there more to this? Was it a place where there could be undergorund training in the mystic arts?
The scale at which the work was undertaken is enormous. It covers twice the area of the Parthenon in Athens and is 1.5 times high, and it entailed removing 200,000 tonnes of rock. It is believed to have taken 7,000 labourers 150 years to complete the project.
Ref. https://www.world-mysteries.com/mpl_11.htm
I am not sure whether they were trying to isolate themselves from non-believers. The article points out that there was fierce competition between the Buddhists and Hinduists to attract potential believers. The underground caves probably were just made as dwelling quarters for inhabitants of the temple, and the reason for being underground was for solace.
Understanding Hindu beliefs helps us understand why this was built. Shiva is the destroyer, one of the three primary gods on the Hindu belief. All other gods are forms of Shive, Vishnu, and Bramha. That being said, if you were to build a temple in the mountains, you would most likely want it to be as close to the god and beautiful enough for the god. You wouldn't want the god to get angry.
I also think it had something to do with protection. I believe that it was built to protect them from invaders that would be other religions and unbelievers, or even just followers of Vishnu or Brahma. It wasn't uncommon to kill everyone in the temple to establish your god as dominant. Being in the mountains, it would be hard to get to. Being stone, it would be impossible to penetrate in that day and age.