Voklina (pronounced voklina — from "oko," meaning "eye") — a deep collapse, a karst sinkhole-gully on the bed of a body of water.
A characteristic formation found in a number of lakes in the Nizhny Novgorod Region, for example, Vad Lake.
From voklinas, intense jets of cold groundwater at temperatures of 4–6 °C surge upward, creating the appearance of a convex "lens" on the lake's surface; powerful underwater currents radiating from the voklina for up to 20 metres prevent the water from freezing even in severe frosts. Vad Lake historically had two voklinas; in 1938, after the monastery dam was restored, one of them collapsed, while the surviving voklina was an enormous underwater funnel with mosaic walls covered in blue-green algae and rare plants. In April 2007, the cave ceiling of the Vad voklina collapsed, sealing the outlet of the underground river Vadok at the lake bed; according to scientists, there had previously been three such caves in freshwater lakes on the planet — in Belgium, France, and Russia — leaving only two after the collapse.[1] According to eyewitness accounts, the cave-in occurred in mid-March 2007: the water in the centre of the lake suddenly began to churn, a muffled thud was heard, and the water quickly turned turbid; a group of Nizhny Novgorod divers who arrived at the lake on 8 April were no longer able to find the cave entrance.[1] The non-freezing window above the voklina and the water's clarity attracted divers from the Nizhny Novgorod Region and other regions; in 2005, the Vadskoy dive centre was opened on the shore of Vad Lake.[2][2][2][2]
Sources
- vglubine.ru — Lake Vad — the March collapse — vGlubine.ru — … (archive)
- putidorogi-nn.ru — Lake Vad (Nizhny Novgorod Region): location, photos, description (archive)