Lake Constance or Bodensee is the 3rd largest lake in Central Europe, located in the Alps on the border between Germany (Baden-Württemberg and Bavaria), Switzerland (Thurgau and St. Galen) and Austria (Province of Vorarlberg). 
There is no universal agreement about the lake where exactly the boundary between the bordering countries lies within the lake. 
According to Switzerland, it runs in the middle, according to Austria, the lake is a joint property (condominium) of all the countries along its shores, 
and Germany does not have a clear position. Area 536 km², volume 48 km³, 
average depth 14 m, maximum 254 m. Length from west to east 63.3 km and maximum width from north to south 14 km. 
Its coastline is 273 km long: Germany – 173 km (northern coast), Switzerland – 72 km (southern coast), Austria – 28 km (southeastern coast).

Mainau  is an island in Lake Constance, Baden-Württemberg, Germany. 
It has been called "Island of Flowers" and "Island of Butterflies". Fürst Nikolaus von Esterházy, who bought Mainau in 1827, 
was the first to appreciate that the island's mild climate favored the cultivation of rare, tropical and subtropical plants and sowed them.
The foundations of the current park were laid by Duke Friedrich I, who acquired Mainau in 1853, who during his many travels enriched the collection with new, exotic flowers, created the Italian rose garden and the greenhouse. After his death, the appearance of the island declined. Handed down by inheritance, from 1927 Mainau became the property of the Swedish royal court.
In 1932, the island was given as a wedding gift to Count Leonard Bernadotte by his father, Prince Wilhelm. Count Bernadotte, whose days are marked more by labor than by an aristocratic lifestyle, settles there, reforests the deserted island, turns it into a park and opens it to visitors. During National Socialism, the count returned to Sweden, and after the Second World War he settled on Mainau, and since then the island has been open to visitors again.