Jökulsárlón (literally "glacial river lagoon") is a large glacial lake in southeast Iceland, on the edge of Vatnajökull National Park. Situated at the head of the Breiðamerkurjökull glacier, it developed into a lake after the glacier started receding from the edge of the Atlantic Ocean. The lake has grown since then at varying rates because of melting of the glaciers. It is now 1.5 kilometres away from the ocean's edge and covers an area of about 18 km2. It recently became the deepest lake in Iceland, at over 248 metres, as glacial retreat extended its boundaries. The size of the lake has increased fourfold since the 1970s. It is considered as one of the natural wonders of Iceland.
The lake developed only about 60 years ago (1948 is mentioned), when the entire area was less than 30 m of glacier, which was only 200-250 m from the Atlantic Ocean, and 3 km away from Vatnajökull. Vatnajökull was at the shore line of the ocean and dropped icebergs into the ocean. However, it started drifting in land rapidly every year leaving deep gorges en route, which got filled with melted water and large chunks of ice. These icebergs gather at the mouth of the lake's shallow exit, melt down into smaller ice blocks and roll out into the sea. In summer, icebergs melt and roll down the channel into the sea. In winter the lake freezes and locks the icebergs in place. Jökulsárlón lake, the "glacier lake", is now reported to have doubled in size in the recent 15-year period. The huge blocks of ice that calve from the edge of Vatnajökull are about 30 metres high which fills the lagoon stocked with icebergs.
Given the current retreat rate of Vatnajökull, it is anticipated that there will likely be a deep fjord where Jökulsárlón is now in the near future.
The icebergs that calve from the glacier edge move towards the river mouth and get entrenched at the bottom. The movement of the icebergs fluctuates with the tide currents, as well as being affected by wind. However, they start floating as icebergs when their size is small enough to drift to the sea. These icebergs are seen in two shades: milky white and bright blue, which depends on the air trapped within the ice and is an interplay of light and ice crystals.