The UNESCO World Heritage Site known as the Defence Line of Amsterdam (in Dutch named Stelling van Amsterdam, ) is a ring of fortifications around Amsterdam. It has 42 forts that are from the centre and lowlands, which can easily be flooded in time of war. The flooding was designed to give a depth of about , too little for boats to cross. Any buildings within of the line had to be made of wood so that they could be burnt and the obstruction removed. The Stelling van Amsterdam was constructed between 1880 and 1920. The invention of the aeroplane and tank made the forts obsolete almost as soon as they were finished. Many of the forts now are under the control of both the town councils and the nature department. They may be visited by the public, and admission is free on Monuments Day, the second Saturday in September. Function The Stelling van Amsterdam was primarily a defensive water line (Dutch: waterlinie). In the event of an enemy attack, large tracts of land around Amsterdam would be inundated with water, preventing the enemy from advancing. Amsterdam would function as a national redoubt or reduit, as the last stronghold of the Netherlands. Forts were built in which roads, railways or dikes crossed through the water line. At such locations, there would be no water to stop the enemy and so the forts were intended to shell the enemy. Construction The law for the construction of the Stelling van Amsterdam was passed in 1874, a few years after the Unification of Germany, which placed a powerful new great power on the eastern border of the Netherlands. During the planning prior to its construction, the design was already obviously outdated by modern technical advances. The invention of the high-explosive shell and the percussion fuze, which allowed ordnance to explode on impact and dislodge brick fortifications easily, necessitated a change from masonry to concrete forts. The Dutch did not have the required experience yet using and building with concrete and so extensive tests had to be performed. Concrete structures were shelled with the heaviest artillery available at that time. Further delays resulted from the fact that the sand foundations had to settle for several years before the forts could be built on them. Only in 1897 could the actual construction finally begin. Service The Stelling van Amsterdam has never seen combat service and