Ertholmene, generally referred to as Christiansø, is a small archipelago located approximately 18 km northeast of Bornholm. Its permanent population is about 100 people and its area is 39 hectares. Ertholmene belongs to Denmark and has the most eastern point of the country.
Ertholmene consists of three main islands: Christiansø (named after King Christian V), Frederiksø (named after King Frederick IV), and Græsholm, and several smaller rocks and skerries. The most famous of the latter are Tat and Østerskær. Christiansø covers 22.3 hectares, Frederiksø 4 and Græsholm 11 hectares.
Only the islands of Christiansø and Frederiksø are inhabited, and Græsholm is a bird sanctuary. The link between Christiansø and Frederiksø is a well-sheltered natural harbor, a pedestrian bridge that moves aside to allow larger ships to enter the port.
The islands are an unincorporated area and do not belong to a municipality or region. They are public property, managed by the Danish Ministry of Defense, which is responsible for tasks normally carried out by municipalities and other civil services in the public sector.
The main sources of income is fishing and tourism. Annually, it is visited by 80,000 tourists, mostly day-trips from Bornholm.
Ertholmene is also a popular destination for yachts.
Bornholm fishermen have used Ertholmene as a temporary shelter since the Middle Ages. The first permanent residents appeared as a result of the Danish-Swedish conflicts at the end of the 17th century.
As Denmark needed a naval base in the middle of the Baltic Sea, a fort was built on Christiansø and Frederiksø in 1684, which served as an outpost for the Danish Navy until 1855. The Church on Christiansø originally served the garrison. The island population peaked in the 1810 census with 829 inhabitants. They were soldiers who were there because of the gunner war.
Many of the historic buildings now serve as living quarters for the local population, and some of them are rented to summer tourists. The outer appearance of the islands has changed very little in over 300 years. Surrounded by thick granite walls with old cannons pointing towards the sea, Christiansø is a picturesque tourist spot seemingly frozen in time.
Formerly part of the fort, Store Tårn has been home to the Christiansø Lighthouse for 200 years, and Frederiksø's small round tower, Lille Tårn, is a museum.
The islands were "discovered" by artists, especially painters, who, captivated by the raw beauty of the islands, began to settle there - even today a small group of them live there permanently. The construction of new buildings and interference with the existing state of the island is prohibited, and all this to keep the island as it was in the times of Christian V - for example, TV antennas are hidden inside the buildings.
Unprecedented nature, picturesque local architecture, remains of old fortifications, and above all, the unique character of the islands attract over 80,000 tourists every year.
Unlike Christiansø and Frederiksø, Graesholmen has never been inhabited.
When in 1684 the plague of the dead was spreading among the men working on the construction of the fortress, the dead were buried in the Plague Cemetery on Graesholmen.
Today, Graesholmen is an ornithological reserve, where auks and guillemots have their breeding grounds here - it is the only such place in Denmark. In the bird sanctuary of Greensholm, around 10,000 pairs of silver gulls and an alk inhabit this only place in Denmark. During spring and autumn, millions of small birds migrate across the Baltic Sea to the breeding areas in the north, while large numbers of migratory birds seek food and rest on Christiansø before continuing their journey.
The island is closed to visitors.