Silfra

Silfra is a crack in the Earth filled with some of the clearest fresh water anywhere: a narrow flooded fissure at the edge of Lake Þingvallavatn in south-west Iceland, opened by the slow separation of the North American and Eurasian tectonic plates. The water is so transparent that boulders far below the surface stand out sharply, and swimmers in the channel seem to hang in mid-air between two walls of dark volcanic rock.

The fissure lies in the Þingvellir valley, a rift along the Mid-Atlantic Ridge where the two plates drift apart by about two centimetres a year. The accumulated strain is released in larger earthquakes roughly once a decade, and each one can widen old cracks or open new ones; Silfra is among the largest and deepest of them. Rocks that fell into the gap have jammed together into underwater caves and chambers. The deepest point lies at about 63 metres, though most of the channel is far shallower. Divers know its sections as the Hall, a maze of caves; the Cathedral, a straight corridor around 100 metres long that can often be seen almost from one end to the other; and the Lagoon, a shallow basin about 120 metres long lined with vivid green algae.

The clarity is the result of a very long journey. The water begins as meltwater from Langjökull, Iceland's second-largest glacier, some 50 kilometres to the north. A few thousand years ago lava from the Skjaldbreiður volcano blocked the river that once carried it to the lake, and since then it has seeped through porous lava, spending thirty to a hundred years underground before rising from springs in the fissure. It stays at about 2–4 °C all year and is clean enough to drink. Life here is sparse, but the lake and its fissures are home to a tiny amphipod, Crymostygius thingvallensis, known from nowhere else, and dwarf Arctic char live in the darker depths.

Silfra lies within Þingvellir National Park, a UNESCO World Heritage Site and the place where Iceland's assembly, the Althing, first met in 930. Sessions were held in the valley until 1798, and the ruins of the stone booths used by the gathering's participants can still be traced there.

Getting there

Silfra is about 40 km north-east of Reykjavík, roughly 45–60 minutes by road. The nearest airport is Reykjavík Domestic Airport (RKV), about 42 km away; international flights use Keflavík International Airport (KEF), about 78 km away in a straight line.