Though they often have complicated beginnings, the world’s most notable bunkers can offer powerful lessons. These didactic structures are witnesses to history—many on this list were built during the Second World War and the Cold War—and speak to the way design and architecture evolve with human need. In the 1960s, the threat of nuclear conflict was always present and saw the development of major, even monumental, structures like the extensive underground galleries built during the Mao era in China or a bunker commissioned by Canada’s prime minister at the height of tensions during the Cold War. In addition to their often astonishing appearances, these complexes are reminders of the scale of the world’s conflicts in the 20th century. Whether converted into museums or simply abandoned, these military sites leave a powerful impression on visitors today. Below, dive into nine of the world’s most notable bunkers.