I spent the majority of the day in Rabat underground. First it was the grotto, then it was the shelters. Malta was part of the Mediterranean and Middle Eastern theatre in World War II, as it was a strategic British post. Hence, it was attacked by both Italy and Germany during World War II for a total of at least 2 years.
Malta is a small country, and during this time, most people left the cities. There were enough shelters for civilians, as the British made it their priority to defend the military installations first. These underground shelters in Rabat are one of the most extensive shelters made, and can currently be visited. The pictures I have below show the current conditions of these shelters, dug out of the Maltese underground.
Don't visit this place if you are claustrophobic. I visited this place alone, and at times, there were labyrinthine paths, which didn't bother me at all, but I can see others getting a little stressed by it. It is cool underneath, so bring a sweater or a jacket if you think you'll be cold.