Probably the most unexpectedly breath-taking sight in Iceland. Pictures don't do it justice. Also, the other large crater nearby looks out-of-this world.
Such a beautiful lake and you can hike around the entire perimeter as well as down to the water's edge. Modest entrance fee can be paid in kronur, Euros, or US dollars.
Bypass the roped path leading down to the water. Use the grass paths to have a more enjoyable (and safer!) decent to the bottom where you don't have to slip on loose gravel.
Worth a quick stop if driving by but given so many other amazing places in Iceland, you could skip it. Especially considering it was one of few (only?) natural sights places that charged admission.
400 ISK Eintritt. Kinder kostenlos. Parkplatz vorhanden, WC nicht. Oben auf einem angelegten Pfad rumlaufen (Länge 800 m) und keine Pflanzen abreissen. Ein Treppe führt runter ans Wasser. Nett.