Lake Thun

Lake Thun stretches 18 km in length and 3 km in width through the Bernese Oberland, at 560 m elevation according to Wikipedia (558 m according to Wikidata). Its shores carry the settlements of Spiez, Thun, Hilterfingen, Oberhofen, Gunten, Sigriswil, Merligen and Neuhaus, home to around 400,000 people. The Kander feeds it; together with Lake Brienz it forms a freshwater reserve supplying drinking water to close to half a million people, and serves as a stopover of pan-European importance for migratory birds.

Its waters, low in phosphorus, hold slow-growing whitefish that have differentiated by habitat and diet — the kropfer (Coregonus alpinus), an endemic species, alongside the brienzlig, the albock and the balchen. Since 2000, more than 40% of these fish have shown unusual congenital and sexual abnormalities; five years of research have not established the cause.

Thun, at the lake's northern end, keeps a medieval town and houses a Historical Museum and an Army Museum. The castles of Spiez, home to the Bubenberg and then the Erlach families, and Oberhofen, held by the knights of Oberhofen and then of Scharnachthal, stand over the shore; the caves of St Beatus, a medieval pilgrimage site, have become a tourist attraction.

Why this place is remarkable

Lake Thun holds together scales rarely found in one place: whitefish that have diversified into distinct species by depth and diet, Ferdinand Hodler, who painted it repeatedly alongside the Stockhorn, and a little-known military legacy — up to 4,600 tonnes of munitions sunk between 1945 and 1964, whose effect on wildlife remains unresolved.

History and origin

The lake's name, like that of the town of Thun, comes from the Celtic dunum, 'fortified hill' or 'stronghold'. Fredegar's chronicle, in the 7th century, records it as in laco Duninse quem Arola flumen influit, 'the lake of Thun into which the Aare flows'; the same text reports that in the year 599 the lake's water boiled and cooked the fish, an episode the author himself calls prodigious.