Arcologies are fully-enclosed areas housing large populations of humans on Earth. The environment on Earth has been rendered unsafe by centuries of industrial-ecological abuse.
Each arcology, built in the 21st century mostly on top of pre-existing cities (and enclosing their historical centers), protects humans from the deadly atmospheric conditions outside while providing a complete and self-contained “world” inside. Most earthlings grow, mature, and die on their planet without ever having seen what used to be known as “the sky”.
(Note: This list consists only of items mentioned in the sources. It may be very incomplete.)