A hypogeum is a pre-Christian underground burial site (a Christian one is generally called a catacomb), and the finest and most famous of these is Hal Saflieni, located in the Valletta suburb of Paola. A visit there has to be planned well in advance, but this is definitely an unmissable and unique site. The building above the hypogeum is an ordinary house in a small street near the church (there are enough signs to point you there), and inside, there is only a small waiting room. The tour starts with an introductory video, and then small groups of no more than 10 people are led underground to enjoy an almost surreal experience. It is quite hard to describe, but you really get the feeling of being 5,000 years in the past and discovering a unique, sacred place. In short, go to the Heritage Malta website and book your tickets now...