The Atoll is a building exclusive to the Aquarion tribe that can connect to cities and other Atolls through water, ocean, and Flooded tiles within a 3 tile radius. Atolls can only be built on aquatic tiles.
Compared to Roads, the Atoll is on average a much more cost-effective way of connecting cities separated by flooded tiles. It is also more effective than Bridges and Ports at connecting coastal cities separated by small bodies of water.
Prior to the fifth Aquarion rework beta, only one Atoll could be built per city.