The Hungarian Mining Museum is built around the only surviving section of a 19th-century coal mine that you can still walk through underground in Hungary. A guided tour takes you 280 metres into the hillside on the original miners' track, with original lamps, drills, ventilation cars, and the rough timber supports that held the seams up.
The above-ground exhibits cover the social history of the Nógrád coal basin, which made Salgótarján a working-class industrial centre from the late 1800s until the mines were exhausted in the late 20th century. The closure is the reason the town has the slightly faded post-industrial feel it does today. A genuinely interesting hour, more so for the underground section than the displays.