These two lakes, Öskjuvatn and Víti, are situated in an enormous crater, around 50 square kilometers, formed in a prehistoric eruption.
The area is still a very active volcanic area, the most recent eruption having been in 1961. The largest historical eruption was in 1875, causing a wave of emigration from Iceland. Seismic activity on the northeastern rim of the caldera has been increasing lately, hinting at the possibility of an upcoming eruption.
The large lake, Öskjuvatn, used to be the deepest lake in Iceland at 220m. However, the famous lagoon, Jökulsárlón in the southeast of Iceland, surpassed it as the deepest two years ago.
The small lake Víti (E. Hell) is popular among tourists for bathing. It was formed in the eruption in 1875 and parts of it are still boiling. If you go there do not take a bath unless there is some wind as carbon dioxide can collect above the surface causing swimmers to pass out. It’s sometimes called the only nudist beach in Iceland.