"This is the biggest active, water-filled thermal water cave and hall in the world," speleologist Sandor Kalinovits, one of the lake's discoverers, said during a tour of the cave below one of Budapest's more affluent residential districts.

The lake, discovered earlier this year, lies in a subterranean hall 86 m (282 ft) long, 27 (89 ft) m wide and 15 m (49 ft) high and belongs to the Janos Molnar cave.

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