Zuid-Holland (South Holland) is the country's most populous province and one of the world's most densely populated areas. The provincial capital is The Hague, which is also the Dutch seat of government, its largest city is Rotterdam, famous for Europe's busiest seaport, the Port of Rotterdam. The whole province is lowland, much of it below the sea level, and there are also some islands. The geography and geology does not support underground structures, neither natural nor artificial. Even cellars tend to fill with ground water. However, the coast had been of strategic importance to the German Army during world War II and so there are numerous bunkers of the Atlantikwall. All listed subterranea are such World War II bunkers.