After visiting Seltun, further north along the detour route towards Reykjavik you will see the huge elongated Kleifarvatn Lake on your right. Be amazed by its eerie still surface and water, which appears like thick oil. To add to the amazement, the beach is black due to volcanic sand. There are numerous hot springs along the beach as you walk, so watch the step.
Legend has it that Kleifarvatn Lake has its own Loch Ness monster... maybe you can catch a glimpse of it on a fine day.