Hagongur is the name of two similar mountains, The Northern-H. and the Southern-H. Their geological structure is Rhyolite and they are cone shaped and steep. They are very prominent, free standing mountains and the distance between them is about 4 km.
South of them is a wide spread lava field called Hagonguhraun. A younger lava field, Trollahraun, covers its southern part. Dams were built in this area in 1998 and a large area disappeared under the water, which is supposed to supply the power stations in the South with more water. A road, built by the contractors, leads all the way up there from the main track across Sprengisandur highland route.