According to National Day Calendar , approximately 6,000 people in North America reside in some kind of underground architecture.
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Montreal, Quebec, Underground City, or la ville souterraine in French, is the largest underground network in the world. Its 32 km (20 mi) of tunnel cover more than 41 city blocks (about 12 km2 (5 sq mi)).
Subterranean London refers to a number of subterranean structures that lie beneath London. The city has been occupied by humans for two millennia. Over time, the capital has acquired a vast number of these structures and spaces, often as a result of war and conflict.