There is little doubt regarding the boundlessly rich and diverse nature of Hiiumaa. It truly has just about everything a sliver of land at these latitudes could conceivably offer.
The sea is shallow around the island with many shoals, with the one called Hiiumadal in the northwest being the best known. The five-metre depth contour line lies several kilometres offshore nearly everywhere around the island.
There are many islets and reefs that just break the waterline – about 200 of them.
The island’s relief is mainly level, but there are hills with steep slopes (in Kõpu, Palade and Tahkuna).
Limestone forms the bedrock of Hiiumaa. In some places, (Sarve, Heltermaa, Kõrgessaare) limestone is close to the surface, but mainly it is under several dozen metres of sediment. Limestone is not considered a considerable natural resource on Hiiumaa.
Limestone is usually covered by calcic luvisols, clays and gravel sands. As a natural resource, the varved clays are most prized. They lie several dozen metres deep in the eastern half of the island. With regard to natural resources, the Käina Bay therapeutic mud should not be overlooked.
Hiiumaa is located in the temperate Atlantic-continental zone, characterized by a warm summer and cool winter. The average air temperature in February is -3.5 : -4.5°C, and in July, +16.5 : +17°C. The average annual temperature is +5.2 : +5.8°C. South and southeast winds are predominant. The average wind speed is 5-6 m/sec, and the top wind speed recorded is 34 m/sec.
Hiiumaa is located in an area of transition from coniferous forests to broad-leaved forests. Hiiumaa’s natural landscapes are dominated by pine forests, marshy deciduous forests, spruce mixed forests and juniper stands, coastal meadows and dunes, bogs and marshes. Hiiumaa’s limestone alvars are very interesting; in these areas a thin layer of soil covers limestone plains.
Hiiumaa is the most forested county in Estonia, with over 70% of the area is covered with forests and shrubs. The central part of the island has extensive marshland, comprising 7% of the island’s area. Thus agricultural land and settlements make up under 20% of the island’s area.
The species-richness of Hiiumaa is noteworthy. It is home to close to 1,000 species of higher plants. Over 50 plant species are under protection, such as yew, ivy, sea holly and fairy flax.
Forests are home to moose, deer, and wild boars. Fox, badgers and lynx are common. The Hiiumaa waters are a habitat for Wendell seal and grey seal herds that are noteworthy for the Baltic Sea.
Migratory bird routes also pass over Hiiumaa. Käina Bay is the most famous of the many nesting sites and rest stops for migratory birds.