How were the caves in china formed? And how did people discover them?

Some of the caves were formed by the nature, while some were carved by people.

For instance, the Mogao Caves were carved by people in the year of 366. The Yungang Grottoes were made in the year of 460. These caves do not need to be discovered, because they were carved just for visiting.

For other caves, like the karst caves, they were found by local people accidentally.