János-hegyi Cave, Natural cave in Budapest, Hungary

János-hegyi Cave is a through-cave formed in Upper Triassic dolomite on the north-western slope of János-hegy in Budapest's 12th district. Its lower entrance, 2 m wide and 1.3 m high at 455 m above sea level, leads into a gently rising corridor that changes direction several times; the upper opening lies at 458 m. The passage widens near the exit into a 4.5 m high pointed dome with two small window-like holes, and its horizontal extent is 11.2 m.

Gusztáv Thirring first mentioned it in his 1900 book, describing it as a simple rock fissure. In 1919 Ottokár Kadić, commissioned by the Hungarian Royal Geological Institute, surveyed the cave with Imre Som and members of the Pannonia Tourist Association's cave research section, producing a plan, a longitudinal section and three cross-sections. His report, published in Barlangkutatás in 1919, gives a length of 17 m and notes the cave is unsuitable for excavation.

The cave's name has shifted repeatedly in the literature: Thirring Gusztáv called it a simple rock fissure in 1900, and it appeared as Jánoshegyi barlang, János-hegyi-sziklaüreg and János-hegyi Átjáró-barlang before settling on János-hegyi-átjáró in 1976. Kadić Ottokár's 1919 survey, assisted by Som Imre and members of the Pannonia Turistaegyesület, produced its first floor plan and cross-sections.

The cave is not gated and can be visited freely without cave equipment; it is easy to walk through. The lower entrance opens at 455 m above sea level beside the asphalt path on the north-western slope of János-hegy, the upper one at 458 m a few metres higher. From Szépjuhászné follow the red-stripe trail, cross the Jánoshegyi út twice, then turn left onto the asphalt road at the third crossing and continue about 250 m. Red cave and green cave marked paths of roughly 500 m and 100 m also lead to it.

The cave is a through-cave: its lower entrance sits at 455 m and its upper exit at 458 m, so walkers pass through rather than turn back. Near the exit the passage widens to 2 m and the ceiling rises into a 4.5 m pointed dome pierced by two small window-like holes opening onto the surface above.