The extensive Katale Khor cave system, rated by locals as the best in western Iran, is 150km south of Zanjan, off the Soltaniyeh–Hamadan road near Garmab. Less touristed than the more famous Ali Sadr caves, which it is thought to eventually join, Katale Khor has 3km open to the public and another 4km available for experienced cavers. There are six recorded levels and a visit takes one to two hours. There's no public transport to the site; hire a taxi.
The easiest approach is to hire a taxi for the day from Zanjan (US$70) and combine it with a visit to Soltaniyeh and the tomb of Ghydar Nabi in Ghydar (Qeydar), either returning to Zanjan or continuing on to Hamadan. Alternatively, take a savari to Soltaniyeh and another to Ghydar, and hire a taxi from there (unless you find a savari to Garmaab). An approach from Bijar via Tarta Deh is also feasible.