Just a bit to the southeast of Fljótstunga on Rte 518, a bright yellow sign marks the turn-off to Arnarvatnsheiði along Rte F578 (rental cars not allowed). Follow the bumpy track for 7km to reach Surtshellir, a dramatic, 2km-long lava tube connected to Stefánshellir , a second tunnel about half the size. You can explore Surtshellir on your own, if you have caving gear.
