A few hundred metres below Somoskő Castle, on the Slovak side of the border, the basalt waterfall is a frozen cascade of hexagonal columns flowing down the hillside in a fan twenty metres wide. It is a textbook example of columnar jointing — basalt lava cooling slowly enough to crack into regular six-sided prisms — and one of the more striking geological sights in this part of central Europe.
Visit on the same trip as the castle; the path drops down to the foot of the formation in a few minutes. The whole area is part of the Cerová vrchovina protected landscape, with marked walking trails extending further into Slovakia for those with the time.