The UNESCO World Heritage Site known as the Defence Line of Amsterdam (in Dutch named Stelling van Amsterdam, pronounced [ˌstɛlɪŋ vɑn ɑmstərˈdɑm]) is a 135-kilometre (84 mi) ring of fortifications around Amsterdam. It has 42 forts that are 10–15 kilometres (6.2–9.3 mi) from the centre and lowlands, which can easily be flooded in time of war. The flooding was designed to give a depth of about 30 centimetres (12 in), too little for boats to cross. Any buildings within 1 kilometre (0.62 mi) of the line had to be made of wood so that they could be burnt and the obstruction removed.