Voted Best Location of Summer 2015! 
Sylvan Island is a floating island that is divided into 5 parts, 4 edges and a Center. The edges represent the different seasons, and are divided by a river. The center is where Sylvan City is and seasons can change.
Ecology
In Sylvan City, the ecology varies depending on the season. The seasonal areas flora is the next:
Spring Zone
On Spring zone, there are many flowers and your stereotypical pink cherry trees. The vegetation is mostly flowers and such, making this place perfect for picnics.
Summer Zone
The summer zone comes mostly with normal trees and some flowers. There are also more lakes and even a beach. Here is a good place to swim (except on the rivers).
Autumn Zone
Autumn has trees with orange leaves that ocassionally fall. There are also sunflowers and people that live here occasionally farm corn. Peacocks are the most common people seen here.
Winter Zone
Winter zone is located by his snow covering the floor. Trees are there and are covered by snow, and there are many pine trees. There are cold and frozen lakes here too. This is the most dangerous of the four seasonal areas due to having very slipery and fragile ice.
Climate
Sylvan City climate also varies depending on the season. The seasonal areas climate is the next:
Spring Zone
Spring zone temperature is balanced between hot and cold, so it is warm. The climate is mostly sunny and overcast. There are also chances that it starts raining when overcast.
Summer Zone
Summer zone temperature is mostly hot. The climate is mostly sunny, with low chances of overcast or rainy weather.
Autumn Zone
Autumn zone temperature is slighty cold. The climate is sunny, but mostly it is partly cloudy, and there are higher chances that it rains. It can also snow, but in rare cases.
Winter Zone
Winter zone temperature is cold all the time, even when sunny. The climate is mostly snowing and overcast, but there can also rain. In rare cases it can be sunny. There is also cold wind, and while snowing, it can obscure vision, making it a dangerous place when it is snowing and there's high wind.
History
Sylvan Island was first a normal island, like the others. However, not many people lived there. One day, the island started to sink, and there were earthquakes because of the island being most water. When it was about to sink completly, someone brought a powerful energy source, and the island started to rise. Thus, it became a floating island.
That source, known as the Sylvan core, is now on a shrine on the center of the island. The source started creating big rivers and dams at the ends of the rivers to prevent the water to flow down. The rivers still flow, but they don't flow off the island. It also made 4 of the parts of the island to become seasonal zones.
Some time later, on the center area, the people left on that area started building homes and other buildings. They also built big bridges to the center of the island. Bridges to go from a seasonal zone to other were made of rope and wood, but still being very resistant. After the people did all that, they started living peacefully on their new home, which they called Sylvan city.
Some thime later, Eggman attacked the island to steal the Sylvan Core, but failed. Some time after that attack, the Sylvan Core Shrine protection was vastly increased. The Core created 4 Seasonal Crystals and sealed the entrace. Now the entrace can only be opened by the 4 Seasonal Crystals, which can be found on dungeons found on each seasonal area. The dungeons walls are indestructible. However, the entrace will also open if the island is on a critical situation. After the entrace opens, a maze must then be crossed, which is also indestrictible. Some areas represent each of the 4 elements: Fire, Ice, Earth and Lightning.
The core did this, as to test the courage of the person colleccting the Crystals, or to access the Core for help if the island (or even the world) is in deep trouble. After that, the Core protection upgrade was finished, and everything went back to normal.
Notable Areas
Sylvan City
Sylvan city is on the center area of the island and it is the only place that can change season. It is currently a medium city that is medieval style, however, technology is here too. It is the only city on the island. The city is surrounded by a big lake, thus having big bridges to go to each seasonal area. Most of the population has good abilities to build, thus being able to build houses or shelters.
Sylvan Core Shrine
On the center of the island there is a shrine with the Sylvan Core. The Sylvan core is the source that makes the island float, the rivers flow and the seasonal areas stay on their seasons. It can also give powers to those that are worthy of it.
Seasonal Zones Mines
The seasonal areas have mines that were used to get certain minerals. These minerals represented different elements.
Spring Zone Mines
On Spring Zone, there are mines which are mostly peaceful and little danger lies inside. Spring Zone unique mineral is Naturite, a mineral made of rock, iron and leaves. It is always covered by Ivory. It represents the Plant element.
Summer Zone Mines
Summer Zone mines are a little warm inside of them, but it has mostly little danger. Summer Zone unique mineral is Flareite, which is a red metal that shines and is somewhat hot to the touch, but it cannot burn anything. It represents the Fire element, so caution is nedded to carry this mineral, since it is hot.
Autumn Zone Mines
On Autumn Zone mines, there are little leaves and some plant thorns, so it is a somewhat dangerous. Autumn Zone unique mineral is Voltite, yellow and gray colored mineral which looks like it has electricity. It represents the Electric element, but it dosen't electrifies those who touch it.
Winter Zone Mines
On Winter Zone mines, there are icicles on various places (some of which are sharp), frozen water (some of which are fragile) and slippery terrain, making it the most dangerous mine. Winter Zone unique mineral is Frostite, a crystaline blue metal which is cold to the touch. It represents the Ice element.