This massive river-cave system 70km north of Hamadan is normally visited as a day trip. It's popular with Iranian tourists, but foreigners usually have mixed feelings, most stemming from the cost (10 times what locals pay) and the perceived commercialisation. However, traversing darkened caverns by paddle boat and walking across subterranean islands is pretty special. Treat it as a cultural experience as well as a scenic one. A tour takes 45 to 90 minutes. Avoid (crowded) weekends and tip your guide.
Ignore the sign saying '$25' and insist on paying in rial – it will work out more cheaply. Ask for the (free) map in your language. The caves are a constant 16°C, so bring more than one layer. The caverns are up to 40m high and the river 14m deep; the system is thought to connect to the Katale Khor system in Zanjan province. There's a Tourist Inn nearby if you feel like staying on and exploring the surrounding area.