The west of the island is a protected nature reserve, with an unusual environment of flads and gloe lakes that provide refuges for fish and birds.

The three largest islands are Halsön, Bredskäret and Södra Björkön, all used as important recreation areas for urban dwellers.

[2] The archipelago, including Halson Island and the surrounding shallow sea, is characterized by an undulating moraine terrain with till surfaces rich in boulders.

[3] WPD Finland Oy has stated plans to build a wind farm in public waters of the Korsnäs municipality.

Halsön is low-lying, with only a small part of the western end more than 10 metres (33 ft) above sea level.

Spruce has been planted, but costs of forestry are high due to the difficulty of access to the island, and moose and deer continue to cause problems.

[2] These are types of brackish lagoon typical of the flat land of the Finnish part of the Kvarken Archipelago.

As the land rises due to post-glacial rebound it becomes a gloe lake, receiving sea water only at high tide.

[11] Species that have been observed include northern lapwing (Vanellus vanellus), common chaffinch (Fringilla coelebs), snow bunting (Plectrophenax nivalis), greylag goose (Ansar anser), barnacle goose (Branta leucopsis) and greater scaup (Aythya marila).

[12] The Kvarken archipelago, which includes Halsön and Molpehällor, has been designated a Unesco World Heritage Site of outstanding natural significance.