The O’Neill Cylinder, designed by Princeton physicist Gerard K. O’Neill, is considerably larger than the other two designs, and is referred to as an “Island 3” or 3rd-generation space colony.
The configuration consists of a pair of cylinders, each 20 miles long and 4 miles in diameter. Each cylinder has three land areas alternating with three windows, and three mirrors that open and close to form a day-night cycle inside. The total land area inside a pair of cylinders is about 500 square miles and can house several million people.
The cylinders are always in pairs which rotate in opposite directions, cancelling out any gyroscopic effect that would otherwise make it difficult to keep them aimed toward the sun. The modules on the large ring structure around the endcap are used for agriculture. Each module could have differing environments ideal for a particular set of food items.
The cylinders are large enough to have weather, which could even be made to change with the seasons, perhaps depending on a colonist vote. Clouds would form at an altitude of 3000 feet. The third image features a vista with ruddy hues caused by a solar eclipse, which would be more common at L5 than on Earth but still infrequent.