Seljalandsfoss is the waterfall you can walk behind. The river drops about sixty metres off the old sea cliff in one narrow fall. The cliff is undercut, so there is a path that goes right around the back of it. You walk in behind the water and look out through it. The path is wet the whole way and you will get soaked. The rock is slick, so watch your feet. It is a short walk.

This is one of the busiest stops in Iceland. It sits right off Highway 1 and the car park is large. We arrived before the tour buses and had it fairly quiet for a while. By the time we left the buses were coming in and there was a queue on the path. Early is better.

A short walk north along the cliff is the other one, Gljúfrabúi. Most people skip it. From the path you only see a dark slot in the rock with a stream coming out. To get to the fall you walk up the stream into the gap, between close rock walls, and it opens into a small space with the waterfall coming down into it. You get wet again.

Two waterfalls, a few hundred metres apart. Everyone stops at the first one. Almost nobody walks the extra five minutes to the second.