A SIB is a Significant Island of Britain. The complete list of SIBs and Siblets comprises over 1100 islands. Only around half
of these are categorised as "Main SIBs". The qualification criteria are to be an island with at least 30 hectares of area or a prominence
of 30m. Islands which are "Vertical SIBs" (sea stacks etc) are excluded from the Main SIBs.