Filling much of the rift plain, Þingvallavatn is Iceland’s largest lake, at 84 sq km. Pure glacial water from Langjökull filters through bedrock for 40km before emerging here. It’s joined by the hot spring Vellankatla, which spouts from beneath the lava field on the northeastern shore. Þingvallavatn is an important refuelling stop for migrating birds (including the great northern diver, barrow’s golden-eye and harlequin duck).
Weirdly, the waters here are full of bleikja (Arctic char) that have been isolated for so long that they’ve evolved into four subspecies. It's possible to submerge into the depths of the the lake with Dive Silfra and potentially see them up close.