Kleifarvatn Geothermal Vents

Kleifarvatn is a volcanic crater lake on the Reykjanes Peninsula, sitting directly above the Mid-Atlantic Ridge where geothermal activity is intense and unpredictable. The lake floor at 5 to 20 metres depth is punctuated by hydrothermal vents that release bubbles of carbon dioxide, hydrogen sulfide, and steam into the water column, warming the sediment immediately around them to temperatures noticeably higher than the main lake body. Chemosynthetic bacteria form orange and white mats around the vent openings, and the chemical plumes are visible as shimmer lines in the ultra-clear water. The lake is freshwater despite its geothermal character, and visibility regularly exceeds 50 metres in calm conditions. A 2000 earthquake drained much of the lake through newly opened fissures, then groundwater slowly refilled it over the following decade — the exposed lakebed geology visible along the shores documents this event. Diving is possible year-round but requires coordination with local operators due to the remote access and geothermal monitoring requirements.