In Norse mythology, Gefjun is an Aesir goddess. She is a virgin and is served by all those who die as virgins. A myth says that Gefjun took a piece of land from the Swedish king Gylfi to make it a Danish island, Seeland. It is written that she achieved this with the help of four oxen, which are in fact the sons she had with a giant. Her name is also spelled Gefjon, Gefn or Gefion.
There are two versions of the myth of Gefjun.
In the Gylfaginning (Gylfi's mystification), King Gylfi ruled over the region now called Sweden. The story goes that he offered a land to a wanderer as a reward for entertaining him. The land would be as large as four oxen could plow in a day and a night. He did not know that the wanderer was an Aesir called Gefjun.
The oxen she used were actually the sons she had had with a giant. They plowed the soil so well and so deeply that a portion of the land broke away from the kingdom and moved westward. The land came to rest at sea, in the Sund. Gefjun left the land there and called it Selund (or Seeland).
The area where the land was torn away became an inland lake, now called Lake Mälaren (also called Lögrinn). However, since modern maps show a similarity between Seeland and the Swedish lake Vänern, it is sometimes identified as the hole left by Gefjun.
The Heimskringla (Saga of the kings of Norway), reports that when Odin considered the future, he knew that his descendants would live and work in the north of the world.
He gave his brothers Vé and Vili the land of Asagarth (not to be confused with Asgard), which was in the territory of the Turks. He himself left and took with him all the priests and a large part of his people. He went first to Gardarike (Russia) and from there to the south, to Saxenland (Germany). He had many sons, he conquered many kingdoms and let his sons rule them.
From Saxenland Odin went north, towards the sea, and settled on an island that took his name (Odensö). This was the present island of Fionie. Odin sent Gefjun to look for land to the northeast, on the other side of the Sund. There Gefjun met Gylfi who promised him as much land as four oxen could plow in one natural day (one day and one night).
Gefjun went north to Jötunheimar where she conceived four sons. She turned them into oxen. They ploughed so much land that a piece was torn off and moved to the west, in a strait (the sund) in front of Odensö creating Selund, the island of Seeland. Gefjun settled there.
Skjöld, Odin's son, took Gefjun as his wife. They settled in Lejre. The place where Gefjun had ploughed is now a lake, Löginn, whose contours correspond to the shapes of Seeland.