Diving in Hungary is based around lakes – check out Lake Balaton, where you can ice dive in the winter. In Budapest, there is an underground thermal lake, a total of 5 km (3.1 miles) with passageways of 82m (269 feet). The water is warm, 20°C in the underlayer and 27°C in the upper. The warmer water flows towards the Alagút-spring.
The spring of Molnár János empties into Lake Malom, which flows after a few hundred feet/meters into the Danube. Divers have found remains of Roman activity at the bottom of Lake Malom, making this an incredible archeological dive if you get lucky.