These old tunnels and caverns had been used for mining fossils. They are now abandoned but plenty of fossils remain and many Pokemon now live here.....maybe even 'extinct' Pokemon. It is rumored a legendary Pokemon wanders the area, defending the caverns from intruders.
"Those who enter, you enter at your own risk. The legendary Regirock roams these caverns, easily defeating even the strongest of trainers without a care. It is driven by one instinct, to protect the Pokemon that live with it."
- A sign outside of the entrance to Fossil Caverns.