The Uros islands at 3810 meters above sea level are just five kilometers west from Puno port. Around 2,000 descendants of the Uros were counted in the 1997 census, although only a few hundred still live on and maintain the islands; most have moved to the mainland. The Uros are a pre-Incan people who live on forty-two self-fashioned floating islands in Lake Titicaca Puno, Peru and Bolivia. They form three main groups: Uru-Chipayas, Uru-Muratos and the Uru-Iruitos.
The Uros use bundles of dried totora reeds to make reed boats (balsas mats), and to make the islands themselves. The larger islands house about ten families, while smaller ones, only about thirty meters wide, house only two or three. The islets are made of totora reeds, which grow in the lake. The dense roots that the plants develop and interweave form a natural layer called Khili (about one to two meters thick) that support the islands. They are anchored with ropes attached to sticks driven into the bottom of the lake. The reeds at the bottoms of the islands rot away fairly quickly, so new reeds are added to the top constantly, about every three months; this is what makes it exciting for tourists when walking on the island. This is especially important in the rainy season when the reeds rot much faster. The islands last about thirty years.
Each step on an island sinks about 2-4" depending on the density of the ground underfoot. As the reeds dry, they break up more and more as they are walked upon. As the reed breaks up and moisture gets to it, it rots, and a new layer has to be added to it. It is a lot of work to maintain the islands. [Wikipedia]