The true Nether experience
This cache was hidden after I went "running" in the lava field. I ran all the way under the mountain (and climbed it partly to have a view over the lava field). On the way back, I could not see the camp and had to rely only on the wooden planks to guide me. When also realizing that it was solid lava under my feet, I immediately remembered playing Minecraft. In Minecraft, the Nether is a part of the world full of floating lava where one has to rely on torches left on his path if he ever wants to get back. I recommend you to follow the path to the mountain. There you can feel its greatness even strongly and enjoy the journey back.
The Herðubreið mountain
Herðubreið is a specific type of volcano formed under the ice cover during the last Glacial. It is sometimes called the queen of mountains. The view of the mountain is breath-taking, especially when you get just under the mountain.
It is apparently very difficult to climb to the top. Nevertheless, there is a trail. If you decide to climb the mountain, you should definitely contact rangers of the park first. They should know about you and can give you vital information. There is a passage where small rocks are falling (especially during melting of the snow), so you should definitely at least have helmets. A part of the trail may be covered by ice so ice axes may also be needed.
During our visit at the end of June 2016, rangers informed us that no one has attempted the ascent that summer. Since we also didn't have any equipment, we decided not to chance our luck.
The road to Herðubreiðarlindir
Herðubreiðarlindir is a small oasis in the dessert with some huts and a camping site. Since it is located in the Highlands of Iceland, you need a good car to get there. For the ones that we rented, the journey was quite rough (especially when crossing rivers).
It is recommended to use some bigger 4x4 car or possibly take a bus.
No off-road in Iceland!
Off-road driving is forbidden in Iceland, especially in the national park. Even though the flat sandy plains may be very tempting for driving, do not make the same mistake we did. We ended up trying to remove our tracks in the sand hoping not to be fined for it.
Maintanance
Since there are no permanent residents in the area, the only chance of maintenance is by travelling geocachers. I believe that it is not a problem for a visitor who finds the cache in a bad condition to fix the problem. Thanks a lot!