The Catacombs of Malta

We all know what catacombs are. Places where the remains of the dead have been interred en masse, but in an organized fashion. The word itself conjures images of rows upon rows of skulls and neatly stacked femurs of countless individuals who lived hundreds, maybe thousands of years ago. The stale smell of damp stone and dirt.
Their usage was popular throughout southern Europe in times of plague and disease to quickly remove the remains of the deceased from the streets.
In the present, these houses of the dead attract the attention of the living individuals who enjoy the macabre.
In most places, they have become tourist attractions and a great source of revenue for smaller communities. Most catacombs will have guided tours with trained tour guides (or security personnel) to walk visitors through designated sections, as without a tour guide, larger catacombs can be nigh impossible to navigate. Additionally, the guides will protect those who've been resting there for millenia from desecration by unruly children and teens who might be on vacation with mommy and daddy.

That being said, not every historical site has the most ironclad security. All it takes is one tired worker after hours or even enough of a bribe, and you may be able to slip inside alone and explore areas not intended for the general public.
Of course, that is not recommended. Not only is it tresspassing, but it's dangerous. Ever wonder why the tour guides always take visitors through the exact same routes?
1: It is unbelievably easy to get lost.
2: Structural issues. Lots of catacombs are thousands of years old, and that means wall or ceiling erosion. One wrong move and the whole thing could come crashing down, burying you with the rest of the inhabitants you came to see.
That's for catacombs that are open to the public in some form or fashion. There are closed catacombs too, or catacombs that have become closed for various reasons.
One such catacomb is on the isle of Malta.
The catacomb itself lies in a cave in Mdina (Citta Vecchia) on the western side of the island. It wasn't necessarily an unpopular tourist location, but wasn't quite "popular" either. From what everyone knew of it, it was relatively small and began accepting remains of the townsfolk in approximately 1686, and was reportedly only in operation as a mausoleum until 1718. Regardless of its popularity with tourists, archeologists in the area began to take a great interest in the site in the mid 1970s for a very specific reason. A recent earthquake had created a fracture in the leftmost chamber's 3 meter thick stone wall revealing an additional section of the cave that had not been seen by any living man for an estimated 4,700 years. It was speculated that a previous civilization had been using the closed off area as a ritual space of some sort, the area was filled with undecipherable pictographic writing embedded on stone tablets and what appeared to be various offerings had been left on a central stone slab. Much work was conducted trying to translate the unknown texts until reasearch abruptly stopped one day in mid April, after which the catacombs were fully closed to both the public and researchers alike.

When asked for details as to why research stopped so suddenly, one retired researcher [name withheld] advised the following:
It was odd. I was new at the time and probably the least experienced of our 12-man crew. Compared to the few academic expeditions I had been on thus far, we seemed to be making steady progress. I would say we were.. maybe just under 50% of the way of breaking down the basic syntax of their written language. I don't really get why we stopped, I didn't see it myself, but reportedly, research stopped because there was a fucking light in there. The light wasn't from the sun and certainly wasn't electrical. It was from something that moved and twisted. It was something that breathed. Jagged and sleek, with whiskers like a catfish. It was light itself. Shining, blinding. It slithered and spoke. It did not touch the ground.
When pressed for further details, the researcher declined.