Bledug Kuwu is an odd geological stop west of Blora: a flat mud-volcano field where grey, salty mud erupts in slow bursts every few minutes. It is not a polished attraction, but the sight is unusual enough to justify the detour if you are already exploring this part of Central Java.
The mud has long been tied to local salt-making and folklore. Go with realistic expectations: the setting is simple, the eruptions are small, and the appeal is in seeing a rare natural process up close.