The earliest UNESCO World Heritage Sites were designated in 1978 during the 2nd session of the World Heritage Committee. At this time, 12 sites were inscribed as the first World Heritage Sites, including natural and cultural heritage locations worldwide.
If the question refers to a specific site, please provide the site's name for precise information on when it became a UNESCO World Heritage Site. Otherwise, for the general context, the UNESCO World Heritage List began in 1978 with these initial 12 sites.
