on march 9, 2017, a strange crack appeared on the ice-laden surface of thingvallavatn, the largest lake in iceland. stretching two kilometers, the crack drew the attention of locals, as well as news organizations from the reykjavik grapevine to the irish news to the uk express. 

“locals baffled by mysterious zig zag lines formed on eerie frozen lake in iceland,” read the uk express. 

“speculation started and there was no shortage of alternative explanations from alien activity and unknown monsters in the lake to conspiracy theories,” commented park official einar ae saemundsen for the irish news. 

locals had reason to be baffled. the crack appeared in a section of the lake that was normally solid in winter. its shape was a long zig-zag that looked much too uniform to appear naturally. but according to geophysicists, there was a logical explanation: finger rafting. 

“a surprising pattern, much like the meshed teeth of a zipper, is frequently seen when floating ice sheets collide,” commented yale geophysics professor john wettlaufer in an article for the yale news. in these rare circumstances, the edges of the ice can become enmeshed, as the edges push alternatively over and under each other, forming “fingers.” 

fifteen years ago, no one would have expected a crack of any size or shape to appear on the lake. “every winter you could guarantee a frost,” said ingunn Ósk Árnadóttir, a ranger at thingvellir national park, which occupies the northern boundary of lake thingvallavatn. “it was even used as transportation. people walked over between farms. it doesn’t happen anymore.” 

with warming temperatures, the ice covering the lake has thinned and narrowed, and the ice is not the only part of the lake in danger of inexorable change. beneath the surface of thingvallavatn, climate change is also taking its toll. for a lake with such a rich and fabled past, thingvallavatn’s future is of special significance for many.

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thingvallavatn “is already quite famous,” Árnadóttir said. “this place is really loved among the nation.” 

merely walking on its banks gives one a sense of its majesty. at 84 square kilometers (nearly 21,000 acres), it is the largest natural lake in iceland. it is exceptionally cold compared to other icelandic lakes. the temperature in its fissures hovers around 2 degrees celsius (about 35 degrees fahrenheit), while at the end of the summer the surface hovers between 10 and 14 degrees celsius (50 to 57 degrees fahrenheit). 

young, porous igneous rock comprises the lake’s catchment area, where the water drains into the lake. this makes the incoming water exceptionally fertile and the lake’s species exceptionally diverse. in fact, the lake’s unique geological history has made it host to a number of species found nowhere else in the world. 

at the end of the last ice age, the lake became isolated from other bodies of water. over the next 10,000 years, the lake’s three fish species (brown trout, arctic charr, and the three-spine stickleback) evolved to fill unique niches in the lake’s many fissures and habitats. the arctic char, for example, evolved into four distinct varieties, each with its own ecological niche: a small benthivore (which eats prey that live on the bottom of the lake), a large benthivore, a planktivore (which eats varieties of plankton), and a piscivore (a species that eats other fish). these species, according to Árnadóttir, are found nowhere else in the world.

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the northern boundary of thingvallavatn is found within thingvellir national park, an area that holds its own mystique. thingvellir’s cliffs, ravines, and waterfalls give the park an otherworldly beauty. it was even cast as a filming location for the television series “game of thrones.” but thingvellir also has a history perhaps more intriguing than fiction. thingvellir was the site of iceland’s annual parliament, or alping, from 930 a.d. to 1798 a.d. at the alping, chieftains would gather from all over iceland to exchange news and write law. leaders would come from the farthest reaches of iceland, sometimes traveling for weeks through valleys and over glacial tongues, to reach thingvellir. 

it wasn’t just political leaders that would gather at the alping. thingvellir was, and still is, considered iceland’s cultural center. during the two weeks of the assembly, a vibrant, market would appear, complete with thousands of merchants, sword-sharpeners, entertainers, and ale-makers. “thingvellir was a meeting place for everyone in iceland, laying the foundation for the language and literature that have been a prominent part of people’s lives right up to the present day,” it says on thingvellir’s website. 

indeed, thingvellir remained a meeting-place long after the alping ended. national celebrations are still held on the grounds, such as the presentation of iceland’s first constitution in 1874, the millennium of the alping in 1930, and the millennium of iceland’s christianity in 2000. therefore, it may come as no surprise that a location of such gravity to icelanders also became the area’s first national park. 

“few icelanders visit thingvellir for the first time without admiring the beauty of the landscape and being reminded of some of the major events that are interwoven into the history of this important place,” wrote university professor guðmundur davíðsson in 1913 as part of a thingvellir’s national park campaign. “these two factors, the historical factors and natural beauty, must stir the feelings of anyone standing in this sacred and legendary place.” 

lake thingvallavatn was integral to thingvellir’s identity as iceland’s gathering site. the location of the alping was chosen for its accessibility and its abundance of freshwater provided by lake thingvallavatn and its tributaries. the assembly gathered on the banks of the Öxará river, a tributary of thingvallavatn. ruins from old assembly sites show how the locations of parliament bent with the will of the river, as a flood or change of course would force the assembly to change location. drekkingarhylur, or the drowning pool, located in thingvallavatn’s tributary, was the retribution site for some women convicted of incest or other moral offenses. the waters of thingvallavatn, then, were privy to every aspect of life at the assembly, from historic gatherings of parliament, to drunken merriment at the market, and to the misery of punishment. indeed “Þingvallavatn,” (thingvallavatn in english), translates to “the lake of the fields of parliament.”

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but the waters of thingvallavatn are rapidly changing. with increasing year-round temperatures, the amounts of algae and nutrients in the water has increased. this algae deteriorates the visibility of the crystal-clear water, and may have an adverse effect on the lake’s vulnerable ecosystem. 

“a disturbance that affected arctic char could eliminate several ecosystem processes,” a paper for ecology and society reports. “where one taxon (a group of biologically similar species, like arctic char) controls an ecosystem process, species change and ecosystem change go hand in hand.” precisely what makes the lake so unique, then, is also what makes it so fragile. 

the lake is also in danger due to the diminishment of its water source, the langjökull glacier. at the end of the last ice age, when the arctic char began to evolve, langjökull touched the edge of lake thingvallavatn. now, however, the tip of langjökull has retreated hundreds of miles from the banks of thingvallavatn. 

with the thingvallavatn’s source so far from its destination, the water must undertake a journey even longer and stranger than the chieftains of the alping. experts say it takes 20 to 30 years for the water to travel from the edge of the glacier to the lake. the water travels underground, reaching a depth of eight miles (deep enough to skim the surface of the molten mantle, before making its way into thingvallavatn). 

but langjökull may soon cease to send water to thingvallavatn. langjökull’s “ice brother,” okjökull, was the first of iceland’s glaciers to be lost to climate change in 2014. when it disappeared, it was renamed ok, as its suffix jokull, which means “glacier” in icelandic, was no longer relevant. langjökull, as iceland’s second-largest glacier, is considerably more voluminous than ok, but may not have a life much longer than its brother. 

glaciologists predict the glacier to disappear in 50 to 100 years. although the lake is also fed by rainfall, langjökull’s retreat may have significant consequences — not for the lake’s ecosystem, but for the body of water itself. as Árnadóttir said, “as [the glacier] retreats, it’s obvious that the lake itself will retreat as well.” 

in the meantime, iceland locals may continue to be baffled, as mysterious cracks continue to form over thin ice.