Pálvölgyi, Szemlőhegyi, and Mátyáshegy caves
There are about 200 accessible caves in Budapest (some open, some not) but Pálvölgyi and Szemlőhegyi are the most popular. Both provide a long network of underground walking trails through narrow passages filled with crystal formations, stalactites, and stalagmites. The Pálvölgyi Cave stretches around 18 miles, making it the longest cave network in Budapest and one of the longest in Hungary. It is also one of the most beautiful. Breathe in at Szemlőhegyi as this cave is known for its mineral-infused air. The caves must be visited on a guided tour that lasts just under an hour. Pálvölgyi and Szemlőhegyi can be visited on a combined ticket. Mátyáshegy cave, just opposite Pálvölgyi, is a bit more intense, with some climbing required and caving equipment provided, so it is best suited to the fit (and the brave).
Adventure-seekers should book a three-hour adventure caving tour (21,000 HUF) with Caving Under Budapest (caving.hu), which offers the chance to climb and crawl through these caves.