The Uros are pre-Incan people who live on self made floating islands in Lake Titicaca on the border of Peru and Bolivia.
The purpose of the island settlements was originally defensive, so if a threat arose they could be moved. Nowadays most of their people live on the mainland but a few hundred still live on and maintain the islands.
Dried totora reeds, which grow in the lake, are used to construct reed boats as well as the islands themselves. The reeds at the bottoms of the islands rot away fairly quickly, so new reeds are added to the top about every three months.