They range from the Great Barrier Reef to Stonehenge, but they are all in some way extraordinary: there are currently 754 places on the World Heritage list of the United Nations Educational Scientific and Cultural Organisation (UNESCO).
To make it onto the UNESCO list a site, monument or landscape has to be judged of 'exceptional universal value' in either cultural or natural terms. But despite their uniqueness, some 33 World Heritage sites are currently judged by UNESCO to be under threat, with a hundred or so more judged to be in an alarming state.
Natural disasters strike indiscriminately, without respect for world heritage. And human activities such as wars, the deliberate destruction of cultural symbols, pillage, pollution, poaching and poorly managed tourism can all do great damage.
In 2003 ESA launched an Open Initiative with UNESCO to use Earth Observation to help developing nations more effectively protect World Heritage sites on their territory. Other space agencies are also set to join the Open Initiative.
The idea behind it is that data from space can be used to monitor these sites, alerting authorities to land use changes that could be putting the sites in danger.
ESA and UNESCO are already co-operating in a joint project using satellite data to protect the habitats of mountain gorillas from human encroachment.
Last update: 5 February 2007