The interior cave was actually a set built in a rented warehouse in Fayetteville, Arkansas. The tight space (AKA virgin tunnel) was built on plywood sheets on a platform a few feet above the ground.
Because of the confined space, most of the crew had to wait outside when filming the final scenes when the shotgun was being fired in the cave. Those few who remained were required to wear hearing protection.
The movies claim to be based on a true story is nothing but a fabrication of a group of cave explorers who discovered a new cave system in Virginia in 2004. No odd findings, monsters or deaths events took place.