Certain areas are designated to protect the best examples of the UK's plants, animals, geological or physiographical features.
Sites of Special Scientific Interest (SSSI) in England, Scotland and Wales and Areas of Special Scientific Interest (ASSI) in Northern Ireland are designated to protect the best examples of the UK's plants, animals or geological features.
They are primarily land-based, but some sites extend below the low water mark. They are set up under national legislation; SSSIs under the Wildlife and Countryside Act 1981 and ASSIs under the Nature Conservation and Amenity Lands (Northern Ireland) 1985.
For more information on guidelines for selecting biological SSSIs see the JNCC.
For more information on guidelines for selecting earth science sites with a view to their long-term conservation see here.