This is one of the major tourist attractions on Malta, but only 60 people a day are allowed to visit it.
The Hal Saflieni Hypogeum in Paola is a World Heritage Site. It is a network of underground passageways and chambers which are thought to be between 5000-6000years old. No-one really knows why it was built but it is thought it may have been an underground temple which later became a cemetery.
Unrestricted visitor access was having a detrimental effect on the Hypogeum so now visitor numbers are strictly limited and all access is by Guided tour only. It is popular and places on the six tours each day soon fill up. In the summer months you need to book at least 4-6 weeks in advance to be sure of a place on the tour you want.
Tickets can only be booked on line through the official site http://booking.heritagemalta.org/