Back when I was young the ground under Galveston Island had tunnels running from the middle of the island all the way to the north end. These tunnels were built during World War II. No one bothered to close off the entrances so we would go exploring. The tunnels have collapse today. Germany had invaded Guernsey island and turned it into one massive fortress. Tunnels ran in every direction. Huge gun emplacements lined the shore. To avoid too many casualties the Allies simply went around the island. Today you can tour these underground complexes. When the world's tallest hotel, the Westin Stamford Hotel in Singapore, was completed in 1986 by the South Korean company Ssang Yong Group which means Double Dragon, North Korea got their shorts in a bind. They wanted to be able to thump their chest and say they could build better taller and more wow that their neighbors to the south. The building was never finished and none of the offices ever occupied.
Here are a few sites built for various purposes and left empty today.