The Ħal Saflieni Hypogeum is an underground prehistoric burial site. Discovered in 1902 during construction works, the site was first excavated by Fr Emmanuel Magri between 1904 and 1906. The Ħal Saflieni Hypogeum is a complex made up of interconnecting rock-cut chambers set on three distinct levels. Earliest remains at the site date back to about 4000BC, and the complex was used over a span of many centuries, up to c. 2500 BC. The Hypogeum was first opened to visitors in 1908 and since then it has been visited by many thousands of people. Unfortunately, this has had a toll on the delicate microclimate of the site which has affected the preservation of the site and the unique red ochre paintings. For this reason, after a conservation project which saw the site closed for 10 years between 1990 and 2000, a new system was established in which only 10 visitors an hour are allowed in for a maximum of 8 hours a day, complemented by an environmental control system which keeps temperature and humidity at required levels.