UNESCO says that for sites to be included on its World Heritage List, they "must be of outstanding universal value and meet at least one out of ten selection criteria" — which include (but are not limited to): having a cultural tradition or to a civilization which is living or which has disappeared, having aesthetic importance to an area, and exhibiting an important interchange of human values.
But just because a site has earned itself a place on the List, it doesn't necessarily mean it will keep it's spot. Armed conflict, natural disaster, severe weather, unplanned urban developments, pollution and more can all play a part in threatening a sites' status.
Below you can find 21 of the sites that have been places on UNESCO's List of World Heritage in Danger. For more information, visit the website.
Río Plátano, Honduras
Sumatran Rainforest, Indonesia
Historic Center of Shakhrisyabz, Uzbekistan
Medieval Monuments in Kosovo, Serbia
Niokolo-Koba National Park, Senegal
Kasubi Tombs, Uganda
East Rennell, Solomon Islands
Humberstone and Santa Laura Saltpeter Works, Chile
Atsinanana Rainforest, Madagascar
Everglades National Park, Florida
Selous Game Reserve, Tanzania
Coro and its Port, Venezuela
Church of the Nativity, Bethlehem, Palestine
Chan Chan, Peru
Abu Mena, Egypt
Air and Ténéré Natural Reserves, Niger
Belize Barrier Reef, Belize