Grænavatn lake is a volcanic crater that got its name from its unusual green color.
The name translates to 'Green Lake’, and as it suggests, the lake is dyed a seafoam green due to the amount of Sulphur in it, as well as its surprising depth of 45 meters (148 ft). The closer to the center of the feature, the more intense the green.
The lake is rather small, and should take around 20 minutes to walk around. However, for a small the lake the depth is unusually deep: Roughly around 45 meters.
Grænavatn is a hydrothermal explosion crater. It erupted only allogenic material, which forms about a 10 m thick layer above the lake level along the south of the crater rim, and spread over the surroundings with diminishing block size with distance. It was followed by a smaller crater forming a recess in the east of the lake. This one produced a sheet of welded lava spatter, which is thickest along the eastern crater rim
Due to its unusual nature and color, the lake has been a source of folklore stories throughout the centuries. Only as recently as the middle of the 16th century an odd creature was spotted coming from the lake.