It is fed by the Holtsá river and separated from the Atlantic Ocean by a narrow strip of sand.

[4] Above Holtsós is a deep freshwater pool called Hellisvatn after the cave that overlooks it, Steinahellir, which was the location of the district assembly for Eyjafjöll from 1818 to 1905.

[5] Hellisvatn does not freeze and therefore attracts many birds in winter when other sources of water are frozen over.

Like other shallow lagoons, Holtsós is prone to overflow its banks when the river feeding it runs high, and it is often necessary to keep the opening to the ocean open against sedimentation to prevent flooding of the adjacent land.

[11][12] The farm called Holt, immediately to the west, was an important centre in the Middle Ages, and Holtsós is probably also identical with the Arnarbælisóss of Njáls saga.