There's an excellent video on YouTube of an American survivalist checking out some new equipment he's bought. He's not a paranormal investigator or anything like that, so he has no reason to fake the footage. It's creepy. There's sounds of chanting in caves underground. I'll try and find a link to it.
That might be a good backstory.
A lot of people, when younger, go to scary places for whatever reason. I remember people going to High Beach and a place called Hangman's hill as older teenagers at night (when they'd get hold of a family member's car) , it was really creepy and yeah, some people would get scared (I would sometimes - lol).
So it's no big surprise someone goofing around in the catacombs gets spooked and feels claustrophobic and gets panicked and drops camera.