On 10 June 1940, Mussolini declared war against the Allies. Less than 12 hours later, Malta suffered its first air-raid attack. The main areas attacked were the Harbour areas around Valletta and the Three Cities. This led a large number of people to leave their homes and seek shelter as far away as possible from the Harbour area. A good number of these refugees arrived in Mellieħa. With the escalation of air-raid attacks, the authorities decided to start digging rock-cut shelters to provide better and safe shelter to all. Rock-cut shelters started to be dug by individuals as well.

The Mellieħa rock-cut shelters are considered amongst the largest that were excavated in Malta during those turbulent years. They have more than 500 metres of excavated passages and rooms. Meant to provide shelter to whole families, the shelters were formed by passages, with various sized rooms on the side. Some of these rooms catered for families, while there was also a maternity section. Another room was well guarded as national treasures were kept there. The shelters had more than one entrance and exit, in order to make sure in the possibility of an entrance being blocked, there would always be a way out for people inside.