When I was in Rabat, I spent most of my time underground. Not only because Rabat was where St. Paul's Grotto was, but also it was the location of St. Paul's Catacombs. This is a huge subterranean system of catacombs, dating from the fourth to the ninth century. Currently, it is managed by Heritage Malta, and my Heritage Pass ticket allowed me to visit this place as well.
The catacombs are marked by small concrete houses at the top. These small houses act as entrances; there is a staircase that descends down into the earth, and then you will find yourself surrounded by tombs and other fixtures related to death. There were plenty of tombs down there, all of them are now cleared of the bodies, but I could only imagine how this place looked and felt like when it was still actively used as a burial place.
If you visit this place, I suggest making use of the free audio guide. There are so many pieces of information that one can learn from it. I learned about the different burial practices during those times. There are so many different terminologies they used, depending on whether the dead bodies are buried covered, uncovered, adult or babies, located on the floor or on the wall, and so forth.
I went in January, so there weren't a lot of people there. But it might not be the case if it were in the middle of summer. It is cool down there, so bring a sweater if you're susceptible to the cold.